|**Stock-Based Compensation**                                         | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    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|**Stock-Based Compensation [Abstract]**                              |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|Stock-Based Compensation                                             | Note 8 –Stock-based Compensation
		
		Under the Company’s 2009 Omnibus Equity Compensation Plan, as amended as of February 27, 2014 (the “2009 Plan”), as approved by the Company’s shareholders to replace the 2004 Equity Compensation Plan (the “2004 Plan”), stock options, stock units, stock awards, stock appreciation rights, dividend equivalents, and other stock-based awards may be granted to employees, non-employee directors, and consultants and advisors.  The 2009 Plan authorizes6,250,000shares for issuance under the plan.  A maximum of3,125,000shares under the 2009 Plan may be issued pursuant to stock awards, stock units and other stock-based awards, subject to adjustment as provided in the 2009 Plan.  During any calendar year, no individual may be granted (i) stock options and stock appreciation rights under the 2009 Plan for more than500,000shares of Company stock in the aggregate or (ii) stock awards, stock units or other stock-based awards under the 2009 Plan for more than 500,000 shares of Company stock in the aggregate, subject to adjustment as provided in the 2009 Plan.  Awards to employees and consultants under the 2009 Plan are made by a committee of the Board of Directors of the Company, except that with respect to awards to the Chief Executive Officer, the committee recommends those awards for approval by the non-employee directors of the Board of Directors.  In the case of awards to non-employee directors, the Board of Directors makes such awards.  AtJune 30, 2017,3,741,035shares were still available for issuance under the 2009 Plan.Nofurther grants may be made under the 2004 Plan.
		
		Performance Share Units– A performance share unit (“PSU”) represents the right to receive a share of the Company’s common stock if specified performance goals are met over the three-year performance period specified in the grant, subject to exceptions through the respective vesting period, generallythreeyears.  Each grantee is granted a target award of PSUs, and may earn between0% and200% of the target amount depending on the Company’s performance against the performance goals.  The following table provides compensation costs for stock-based compensation related to PSUs:
		﻿
		﻿﻿﻿
				
				Three Months Ended
				
				Six Months Ended﻿
				
				June 30,
				
				June 30,﻿
				
				2017
				
				2016
				
				2017
				
				2016﻿Stock-based compensation within operations and maintenance expenses
				
				$
				971
				
				$
				967
				
				$
				1,841
				
				$
				1,492﻿Income tax benefit
				
				
				394
				
				
				392
				
				
				747
				
				
				601
		
		The following table summarizes the PSU transactions for thesixmonths endedJune 30, 2017:
		﻿
		﻿﻿﻿
				
				
				Number
				
				Weighted﻿
				
				
				of
				
				Average﻿
				
				
				Share Units
				
				Fair Value﻿Nonvested share units at beginning of period
				
				
				476,896
				
				$
				27.96﻿Granted
				
				
				125,202
				
				
				30.79﻿Performance criteria adjustment
				
				
				(64,034)
				
				
				27.75﻿Forfeited
				
				
				(12,915)
				
				
				28.01﻿Share units vested in prior period and issued in current period
				
				
				32,400
				
				
				25.31﻿Share units issued
				
				
				(125,999)
				
				
				36.37﻿Nonvested share units at end of period
				
				
				431,550
				
				$
				26.16﻿
		
		
		
		A portion of the fair value of PSUs was estimated at the grant date based on the probability of satisfying the market-based conditions using the Monte Carlo valuation method, which assesses probabilities of various outcomes of market conditions.  The other portion of the fair value of the PSUs is based on the fair market value of the Company’s stock at the grant date, regardless of whether the market-based condition is satisfied.  The per unit weighted-average fair value at the date of grant for PSUs granted during thesixmonths endedJune 30, 2017 and 2016 was$30.79and $28.89, respectively.  The fair value of each PSU grant is amortized monthly into compensation expense on a straight-line basis over their respective vesting periods, generally36months.  The accrual of compensation costs is based on our estimate of the final expected value of the award, and is adjusted as required for the portion based on the performance-based condition.  The Company assumes that forfeitures will be minimal, and recognizes forfeitures as they occur, which results in a reduction in compensation expense.  As the payout of the PSUs includes dividend equivalents, no separate dividend yield assumption is required in calculating the fair value of the PSUs.  The recording of compensation expense for PSUs has no impact on net cash flows.
		﻿
		Restricted Stock Units–A restricted stock unit (“RSU”) represents the right to receive a share of the Company’s common stock.  RSUs are eligible to be earned at the end of a specified restricted period, generallythreeyears, beginning on the date of grant.  The Company assumes that forfeitures will be minimal, and recognizes forfeitures as they occur, which results in a reduction in compensation expense.  As the payout of the RSUs includes dividend equivalents, no separate dividend yield assumption is required in calculating the fair value of the RSUs.  The following table provides the compensation cost and income tax benefit for stock-based compensation related to RSUs:
		﻿﻿﻿
				
				Three Months Ended
				
				Six Months Ended﻿
				
				June 30,
				
				June 30,﻿
				
				2017
				
				2016
				
				2017
				
				2016﻿Stock-based compensation within operations and maintenance expenses
				
				$
				322
				
				$
				296
				
				$
				603
				
				$
				464﻿Income tax benefit
				
				
				133
				
				
				122
				
				
				249
				
				
				191
		
		﻿
		The following table summarizes the RSU transactions for thesixmonths endedJune 30, 2017:
		﻿
		﻿﻿﻿
				
				
				Number
				
				Weighted﻿
				
				
				of
				
				Average﻿
				
				
				Stock Units
				
				Fair Value﻿Nonvested stock units at beginning of period
				
				
				109,273
				
				$
				28.48﻿Granted
				
				
				41,293
				
				
				30.37﻿Stock units vested and issued
				
				
				(26,279)
				
				
				26.12﻿Forfeited
				
				
				(2,287)
				
				
				30.52﻿Nonvested stock units at end of period
				
				
				122,000
				
				$
				29.59
		
		﻿
		The per unit weighted-average fair value at the date of grant for RSUs granted during thesixmonths endedJune 30, 2017 and 2016 was $30.37and $32.09, respectively.
		﻿
		Stock Options– A stock option represents the option to purchase a number of shares of common stock of the Company as specified in the stock option grant agreement at the exercise price per share as determined by the closing market price of our common stock on the grant date.  Stock options are exercisable ininstallmentsof33%annually, startingoneyear from the grant date and expire10years from the grant date.  The fair value of each stock option is amortized into compensation expense using the graded-vesting method, which results in the recognition of compensation costs over the requisite service period for each separately vesting tranche of the stock options as though the stock options were, in substance, multiple stock option grants.  The following table provides the compensation cost and income tax benefit for stock-based compensation related to stock options:
		﻿﻿﻿
				
				Three Months Ended
				
				Six Months Ended﻿
				
				June 30,
				
				
				June 30,﻿
				
				2017
				
				2016
				
				2017
				
				2016﻿Stock-based compensation within operations and maintenance expenses
				
				$
				74
				
				$
				-
				
				$
				104
				
				$
				-﻿Income tax benefit
				
				
				33
				
				
				18
				
				
				125
				
				
				234﻿
		The fair value of options was estimated at the grant date using the Black-Scholes option-pricing model.  The following assumptions were used in the application of this valuation model:
		﻿
		﻿﻿
				2017﻿Expected term (years)
				5.45﻿Risk-free interest rate
				2.01%﻿Expected volatility
				17.7%﻿Dividend yield
				2.51%﻿Grant date fair value per option
				$       4.07
		﻿
		Historical information was the principal basis for the selection of the expected term and dividend yield.  The expected volatility is based on a weighted-average combination of historical and implied volatilities over a time period that approximates the expected term of the option.  The risk-free interest rate was selected based upon the U.S. Treasury yield curve in effect at the time of grant for the expected term of the option.
		﻿
		For thesixmonths endedJune 30, 2016, there werenocompensation costs for stock-based compensation related to stock options, as the previous stock option grant that occurred in 2010 became fully amortized in 2013.  Additionally, there werenostock options granted during thesixmonths endedJune 30, 2016.
		﻿
		The following table summarizes stock option transactions for thesixmonths endedJune 30, 2017:
		﻿﻿﻿
				
				
				
				Weighted
				
				Weighted﻿
				
				
				
				Average
				
				Average
				
				Aggregate﻿
				
				
				
				Exercise
				
				Remaining
				
				Intrinsic﻿
				
				Shares
				
				Price
				
				Life (years)
				
				Value﻿Outstanding at beginning of period
				
				427,335
				
				$
				15.55﻿Granted
				
				120,127
				
				
				30.47﻿Forfeited
				
				(1,053)
				
				
				30.47﻿Expired / Cancelled
				
				(2,812)
				
				
				14.26﻿Exercised
				
				(140,550)
				
				
				16.56﻿Outstanding at end of period
				
				403,047
				
				$
				19.62
				
				4.0
				
				$
				5,514﻿﻿Exercisable at end of period
				
				283,973
				
				$
				15.07
				
				1.6
				
				$
				5,177
		
		﻿
		Stock Awards–   Stock awards represent the issuance of our common stock, without restriction.  The issuance of stock awards results in compensation expense which is equal to the fair market value of the stock on the grant date, and is expensed immediately upon grant.  The following table provides the compensation cost and income tax benefit for stock-based compensation related to stock awards:
		﻿
		﻿
		﻿
		﻿﻿﻿
				
				Three Months Ended
				
				Six Months Ended﻿
				
				June 30,
				
				June 30,﻿
				
				2017
				
				2016
				
				2017
				
				2016﻿Stock-based compensation within operations and maintenance expenses
				
				$
				131
				
				$
				131
				
				$
				262
				
				$
				244﻿Income tax benefit
				
				
				54
				
				
				54
				
				
				109
				
				
				101
		﻿
		The following table summarizes stock award transactions for thesixmonths endedJune 30, 2017:
		﻿
		﻿
		﻿﻿﻿
				
				Number
				
				Weighted﻿
				
				of
				
				Average﻿
				
				Stock Awards
				
				Fair Value﻿Nonvested stock awards at beginning of period
				
				-
				
				$
				-﻿Granted
				
				8,001
				
				
				32.79﻿Vested
				
				(8,001)
				
				
				32.79﻿Nonvested stock awards at end of period
				
				-
				
				$
				-
		﻿
		The per unit weighted-average fair value at the date of grant for stock awards granted during thesixmonths endedJune 30, 2017 and 2016 was$32.79and$33.64, respectively.
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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     
|﻿Stock-based compensation within operations and maintenance expenses |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                
|﻿Income tax benefit                                                  |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   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                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               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