| **Investment Securities (Tables)**                                                                                                              | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| **Investments, Debt and Equity Securities [Abstract]**                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Amortized Cost and Approximate Fair Values of Securities                                                                                        | The amortized cost, gross unrealized gains, gross unrealized losses and approximate fair values of the investment securities at the dates indicated were:  AmortizedCostGross Unrealized GainsGross Unrealized LossesFairValueAvailable for sale at March 31, 2014    U.S. Treasury$15,918$79$14$15,983U.S. Government-sponsored agency securities3,355273,382State and municipal244,2986,2911,451249,138U.S. Government-sponsored mortgage-backed securities326,3294,869661330,537Corporate obligations6,3614282,0284,761Equity securities1,706  1,706Total available for sale597,96711,6944,154605,507Held to maturity at March 31, 2014    State and municipal144,7871,97728146,736U.S. Government-sponsored mortgage-backed securities399,6836,8441,642404,885Total held to maturity544,4708,8211,670551,621Total Investment Securities$1,142,437$20,515$5,824$1,157,128 AmortizedCostGross Unrealized GainsGross Unrealized LossesFairValueAvailable for sale at December 31, 2013    U.S. Treasury$15,914 $80 $21 $15,973U.S. Government-sponsored agency securities3,55012173,545State and municipal231,0053,8783,896230,987U.S. Government-sponsored mortgage-backed securities279,2993,9261,973281,252Corporate obligations6,374 3,6362,738Equity securities1,706  1,706Total available for sale537,8487,8969,543536,201Held to maturity at December 31, 2013    State and municipal145,9416291145,912U.S. Government-sponsored mortgage-backed securities413,4375,2203,722414,935Total held to maturity559,3785,2823,813560,847Total Investment Securities$1,097,226$13,178$13,356$1,097,048 |
| Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity                          | The amortized cost and fair value of available for sale securities and held to maturity securities at March 31, 2014, by contractual maturity, are shown below.  Expected maturities will differ from contractual maturities because issuers may have the right to call or prepay obligations with or without call or prepayment penalties. Available for SaleHeld to Maturity AmortizedCostFairValueAmortizedCostFairValueMaturity Distribution at March 31, 2014:    Due in one year or less$11,621$11,769$3,867$3,867Due after one through five years27,95428,49617,37817,633Due after five through ten years47,65748,83078,68879,407Due after ten years182,700184,16944,85445,829 $269,932$273,264$144,787$146,736U.S. Government-sponsored mortgage-backed securities326,329330,537399,683404,885Equity securities1,7061,706Total Investment Securities$597,967$605,507$544,470$551,621                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
| Gross Gains and Losses on Sales and Redemptions of Available for Sale Securities and Other Than Temporary Impairment Losses Recognized          | Gross gains and losses on the sales and redemptions of available for sale securities, and other-than-temporary impairment (“OTTI”) losses recognized for the three months ended March 31, 2014 and 2013 are shown below. Three Months EndedMarch 31,20142013Sales and Redemptions of Available for Sale Securities:  Gross gains$581$248Gross lossesOther-than-temporary impairment losses                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Investments' Gross Unrealized Losses and Fair Value Aggregated by Investment Category and Length of Time in Continuous Unrealized Loss Position | The following table shows the Corporation’s investments’ gross unrealized losses and fair value, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position at March 31, 2014, and December 31, 2013:  Less than12 Months12 Months or LongerTotal FairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesTemporarily Impaired Investment      Securities at March 31, 2014      U.S. Treasury$4,889$14$4,889$14State and municipal59,0271,410$1,028$6960,0551,479U.S. Government-sponsored mortgage-backed securities255,2041,76214,098541269,3022,303Corporate obligations  3,1902,0283,1902,028Total Temporarily Impaired Investment Securities$319,120$3,186$18,316$2,638$337,436$5,824   Less than12 Months12 Months or LongerTotal FairValueGrossUnrealized LossesFairValueGrossUnrealized LossesFairValueGrossUnrealized LossesTemporarily Impaired Investment      Securities at December 31, 2013      U.S. Treasury$4,875$21$4,875$21U.S. Government-sponsored agency securities3,433173,43317State and municipal129,1093,931$767$56129,8763,987U.S. Government-sponsored mortgage-backed securities330,9145,1635,323532336,2375,695Corporate obligations  2,7113,6362,7113,636Total Temporarily Impaired Investment Securities$468,331$9,132$8,801$4,224$477,132$13,356                                                                                                                                   |
| Investments in Debt and Equity Securities Reported Less than Historical Cost                                                                    | Certain investments in debt and equity securities are reported in the financial statements at an amount less than their historical cost as indicated in the table below.March 31, 2014December 31, 2013Investments reported at less than historical cost:  Historical cost$343,258$490,488Fair value$337,436$477,132Percent of the Corporation's available for sale and held to maturity portfolio29.3%43.6%                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| Debt Securities Credit Loss Recognized in Income, and Other Losses Recorded in Other Comprehensive Income                                       | The following table provides information about those securities for which only a credit loss was recognized in income and other losses were recorded in other comprehensive income.  AccumulatedCredit Losses in2014AccumulatedCredit Losses in2013Credit losses on debt securities held:  Balance, January 1$11,355$11,355Additions related to other-than-temporary losses not previously recognizedBalance, March 31$11,355$11,355                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |

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Tabular disclosure of the amount of other than temporary impairment (OTTI) related to credit losses recognized in earnings including: (a) the beginning balance of the amount related to credit losses on debt securities held by the investor at the beginning of the period for which a portion of an OTTI was recognized in other comprehensive income; (b) additions for the amount related to the credit loss for which an OTTI was not previously recognized; (c) reductions for securities sold during the period (realized); (d) reductions for securities for which the amount previously recognized in other comprehensive income was recognized in earnings because the investor intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost basis; (e) additional increases to the amount related to the credit loss for which an OTTI was previously recognized when the investor does not intend to sell the security and it is not more likely than not that the investor will be required to sell the security before recovery of its amortized cost basis; (f) reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security; and (g) the ending balance of the amount related to credit losses on debt securities held by the entity at the end of the period for which a portion of an OTTI was recognized in other comprehensive income.
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