| **Investment Securities (Tables)**                                                                                                                                                           | 9 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
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| **Investments, Debt and Equity Securities [Abstract]**                                                                                                                                       |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
| Schedule of amortized cost, gross unrealized gains, gross unrealized losses and approximate fair value of investment securities                                                              | The amortized cost, gross unrealized gains and losses and approximate fair value of the investment securities portfolio at the dates indicated were:  AmortizedCostGross Unrealized GainsGross Unrealized LossesFairValueAvailable for sale at September 30, 2015    U.S. Government-sponsored agency securities$100$5$105State and municipal289,99811,617$497301,118U.S. Government-sponsored mortgage-backed securities287,3177,63573294,879Corporate obligations3131Equity securities1,7061,706Total available for sale579,15219,257570597,839Held to maturity at September 30, 2015    State and municipal218,7945,707166224,335U.S. Government-sponsored mortgage-backed securities392,16011,566213403,513Total held to maturity610,95417,273379627,848Total Investment Securities$1,190,106$36,530$949$1,225,687  AmortizedCostGross Unrealized GainsGross Unrealized LossesFairValueAvailable for sale at December 31, 2014    U.S. Government-sponsored agency securities$100$9$109State and municipal216,91511,801$123228,593U.S. Government-sponsored mortgage-backed securities310,4608,771127319,104Corporate obligations3131Equity securities1,7061,706Total available for sale529,21220,581250549,543Held to maturity at December 31, 2014    State and municipal204,4435,71696210,063U.S. Government-sponsored mortgage-backed securities426,64511,527512437,660Total held to maturity631,08817,243608647,723Total Investment Securities$1,160,300$37,824$858$1,197,266                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| Schedule of amortized cost and fair value of available for sale securities and held to maturity securities                                                                                   | The amortized cost and fair value of available for sale and held to maturity securities at September 30, 2015, 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 Amortized CostFair ValueAmortized CostFair ValueMaturity Distribution at September 30, 2015:    Due in one year or less$3,661$3,715$5,521$5,533Due after one through five years13,26513,85424,07625,086Due after five through ten years51,07053,34781,28683,355Due after ten years222,133230,338107,911110,361 $290,129$301,254$218,794$224,335U.S. Government-sponsored mortgage-backed securities287,317294,879392,160403,513Equity securities1,7061,706Total Investment Securities$579,152$597,839$610,954$627,848 Available for Sale Held to Maturity Amortized Cost Fair Value Amortized Cost Fair ValueMaturity Distribution at December 31, 2014       Due in one year or less$3,127 $3,153 $6,258 $6,329Due after one through five years9,565 9,840 18,440 18,930Due after five through ten years48,675 50,889 85,997 87,903Due after ten years155,679 164,851 93,748 96,901 $217,046 $228,733 $204,443 $210,063U.S. Government-sponsored mortgage-backed securities310,460 319,104 426,645 437,660Equity securities1,706 1,706    Total Investment Securities$529,212 $549,543 $631,088 $647,723                                                                                                                                                                                                                                                                                                  |
| Schedule of gross gains on sales and redemptions of available for sale securities                                                                                                            | Gross gains on the sales and redemptions of available for sale securities for the three and nine months ended September 30, 2015, and 2014 are shown below. Three Months EndedSeptember 30,Nine Months EndedSeptember 30, 2015201420152014Sales and Redemptions of Available for Sale Securities:    Gross gains$1,115$910$2,147$2,335Gross losses100Other-than-temporary impairment losses                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Schedule of investment securities with unrealized losses, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position | The following table shows investments securities with unrealized losses, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position at September 30, 2015, and December 31, 2014:  Less than12 Months12 Months or LongerTotal FairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesTemporarily Impaired Available for Sale Securities at September 30, 2015      State and municipal$36,702$497$36,702$497U.S. Government-sponsored mortgage-backed securities6,4828$2,125$658,60773Total Temporarily Impaired Available for Sale Securities43,1845052,1256545,309570Temporarily Impaired Held to Maturity Securities at September 30, 2015      State and municipal13,7111441,7162215,427166U.S. Government-sponsored mortgage-backed securities20,7524013,49117334,243213Total Temporarily Impaired Held to Maturity Securities34,46318415,20719549,670379Total Temporarily Impaired Investment Securities$77,647$689$17,332$260$94,979$949  Less than12 Months12 Months or LongerTotal FairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesTemporarily Impaired Available for Sale Securities at December 31, 2014      State and municipal$1,256$7$9,850$116$11,106$123U.S. Government-sponsored mortgage-backed securities2,186135,4471147,633127Total Temporarily Impaired Available for Sale Securities3,4422015,29723018,739250Temporarily Impaired Held to Maturity Securities at December 31, 2014      State and municipal5,119962505,36996U.S. Government-sponsored mortgage-backed securities9,7918238,49143048,282512Total Temporarily Impaired Held to Maturity Securities14,91017838,74143053,651608Total Temporarily Impaired Investment Securities$18,352$198$54,038$660$72,390$858 |
| Schedule of investments in debt and equity securities reported in the financial statements at an amount less than their 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.September 30, 2015December 31, 2014Investments reported at less than historical cost:  Historical cost$95,928$73,249Fair value$94,979$72,390Percent of the Corporation's available for sale and held to maturity portfolio7.9%6.1%                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
| Schedule of securities for which only a credit loss was recognized in income and other losses were 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 in2015AccumulatedCredit Losses in2014Credit losses on debt securities held:  Balance, January 1$500$11,355Reductions for previous other-than-temporary losses realized on securities sold during the year(500)—Balance, September 30$—$11,355                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |

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Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
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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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