|**Net Income Per Common Share**                                              | 6 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             
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|**Net Income Per Common Share [Abstract]**                                   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|Net Income Per Common Share                                                  | Note 7 –Net Income per Common Share
		
		Basic net income per common share is based on the weighted average number of common shares outstanding.  Diluted net income per common share is based on the weighted average number of common shares outstanding and potentially dilutive shares.  The dilutive effect of employee stock-based compensation is included in the computation of diluted net income per common share.  The dilutive effect of stock-based compensation is calculated using the treasury stock method and expected proceeds upon exercise or issuance of the stock-based compensation.  The treasury stock method assumes that the proceeds from stock-based compensation are used to purchase the Company’s common stock at the average market price during the period.  The following table summarizes the shares, in thousands, used in computing basic and diluted net income per common share:
		﻿
		﻿﻿﻿
				
				Three Months Ended
				
				Six Months Ended﻿
				
				June 30,
				
				June 30,﻿
				
				2017
				
				2016
				
				2017
				
				2016﻿Average common shares outstanding during the period for basic computation
				
				177,609
				
				177,288
				
				177,545
				
				177,196﻿Dilutive effect of employee stock-based compensation
				
				436
				
				796
				
				497
				
				724﻿Average common shares outstanding during the period for diluted computation
				
				178,045
				
				178,084
				
				178,042
				
				177,920﻿
		﻿
		For the threeand sixmonths endedJune 30, 2017 and 2016, all of the Company’s employee stock options were included in the calculations of diluted net income per share as the calculated cost to exercise the stock options was less than the average market price of the Company’s common stock during these periods.
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|﻿Average common shares outstanding during the period for basic computation   |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|﻿Dilutive effect of employee stock-based compensation                        |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
|﻿Average common shares outstanding during the period for diluted computation |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            
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