| **Investment Securities (Tables)**                                                                                                                                                           | 3 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 March 31, 2016    U.S. Government-sponsored agency securities$100$3$103State and municipal312,51617,711$107330,120U.S. Government-sponsored mortgage-backed securities295,4437,18340302,586Corporate obligations3131Equity securities3,882633,945Total available for sale611,97224,960147636,785Held to maturity at March 31, 2016    Federal agencies13,978 22  14,000State and municipal218,3749,24449227,569U.S. Government-sponsored mortgage-backed securities401,64012,26125413,876Total held to maturity633,99221,52774655,445Total Investment Securities$1,245,964$46,487$221$1,292,230  AmortizedCostGross Unrealized GainsGross Unrealized LossesFairValueAvailable for sale at December 31, 2015    U.S. Government-sponsored agency securities$100$4$104State and municipal291,73014,241$60305,911U.S. Government-sponsored mortgage-backed securities342,5504,234518346,266Corporate obligations3131Equity securities3,9123,912Certificates of deposit2,176   2,176Total available for sale640,49918,479578658,400Held to maturity at December 31, 2015    State and municipal219,7676,98215226,734U.S. Government-sponsored mortgage-backed securities398,8327,601787405,646Total held to maturity618,59914,583802632,380Total Investment Securities$1,259,098$33,062$1,380$1,290,780                                                                                                                                                                                                                                                                                                        |
| 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 March 31, 2016, 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 March 31, 2016:    Due in one year or less$3,963$3,990$2,533$2,541Due after one through five years17,17518,08732,52734,282Due after five through ten years54,38757,31090,43492,992Due after ten years237,122250,867106,858111,754 $312,647$330,254$232,352$241,569U.S. Government-sponsored mortgage-backed securities295,443302,586401,640413,876Equity securities3,8823,945Total Investment Securities$611,972$636,785$633,992$655,445 Available for Sale Held to Maturity Amortized Cost Fair Value Amortized Cost Fair ValueMaturity Distribution at December 31, 2015       Due in one year or less$4,658 $4,704 $4,144 $4,148Due after one through five years13,725 14,295 28,054 29,175Due after five through ten years52,878 55,375 81,483 83,646Due after ten years220,600 231,672 106,086 109,765 $291,861 $306,046 $219,767 $226,734U.S. Government-sponsored mortgage-backed securities342,550 346,266 398,832 405,646Equity securities3,912 3,912  Certificates of deposit2,176 2,176  Total Investment Securities$640,499 $658,400 $618,599 $632,380                                                                                                                                                                                                                  |
| 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 months ended March 31, 2016 and 2015 are shown below. Three Months EndedMarch 31,20162015Sales and Redemptions of Available for Sale Securities:  Gross gains$997$1,025Gross lossesOther-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 investment securities with unrealized losses, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position at March 31, 2016, and December 31, 2015: Less than12 Months12 Months or LongerTotal FairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesTemporarily Impaired Available for Sale Securities at March 31, 2016      State and municipal$8,375$98$1,197$9$9,572$107U.S. Government-sponsored mortgage-backed securities1,1315$3,845$354,97640Total Temporarily Impaired Available for Sale Securities9,5061035,0424414,548147Temporarily Impaired Held to Maturity Securities at March 31, 2016      State and municipal3,1794867313,85249U.S. Government-sponsored mortgage-backed securities4,784254,78425Total Temporarily Impaired Held to Maturity Securities3,179485,457268,63674Total Temporarily Impaired Investment Securities$12,685$151$10,499$70$23,184$221  Less than12 Months12 Months or LongerTotal FairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesFairValueGrossUnrealizedLossesTemporarily Impaired Available for Sale Securities at December 31, 2015      State and municipal$7,558$60$7,558$60U.S. Government-sponsored mortgage-backed securities83,3964452,1017385,497518Total Temporarily Impaired Available for Sale Securities90,9545052,1017393,055578Temporarily Impaired Held to Maturity Securities at December 31, 2015      State and municipal1,982151,98215U.S. Government-sponsored mortgage-backed securities69,64151912,90626882,547787Total Temporarily Impaired Held to Maturity Securities69,64151914,88828384,529802Total Temporarily Impaired Investment Securities$160,595$1,024$16,989$356$177,584$1,380 |
| 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.March 31, 2016December 31, 2015Investments reported at less than historical cost:  Historical cost$23,406$178,964Fair value$23,184$177,584Percent of the Corporation's available for sale and held to maturity portfolio1.8%13.9%                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
| 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 in2016AccumulatedCredit Losses in2015Credit losses on debt securities held:  Balance, January 1$500Reductions for previous other-than-temporary losses realized on securities sold during the year(500)Balance, March 31$—$—                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |

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Tabular disclosure of fair value of investments in debt and equity securities in an unrealized loss position categorized neither as held-to-maturity nor trading securities.
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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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