| **Investment Securities (Tables)**                                                                                                              | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| **Investments, Debt and Equity Securities [Abstract]**                                                                                          |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       |
| Schedule of Amortized Cost, Gross Unrealized Gains, Gross Unrealized Losses and Approximate Market Value of Securities                          | The amortized cost, gross unrealized gains, gross unrealized losses and approximate market value of the Corporation's investment securities at the dates indicated were: Amortized CostGross Unrealized GainsGross Unrealized LossesFair ValueAvailable for sale at December 31, 2016       U.S. Government-sponsored agency securities$100$100State and municipal360,779$8,443$5,564363,658U.S. Government-sponsored mortgage-backed securities313,4591,9043,071312,292Corporate obligations3131Equity securities21,8201,03920,781Total available for sale696,18910,3479,674696,862Held to maturity at December 31, 2016 U.S. Government-sponsored agency securities22,619   479 22,140State and municipal224,8113,1361,796226,151U.S. Government-sponsored mortgage-backed securities360,2134,9561,527363,642Total held to maturity607,6438,0923,802611,933Total Investment Securities$1,303,832$18,439$13,476$1,308,795        Available for sale at December 31, 2015       U.S. Government-sponsored agency securities$100 $4  $104State and municipal291,730 14,241 $60 305,911U.S. Government-sponsored mortgage-backed securities342,550 4,234 518 346,266Corporate obligations31   31Equity securities3,912   3,912Certificates of deposit2,176     2,176Total available for sale640,499 18,479 578 658,400Held to maturity at December 31, 2015       State and municipal219,767 6,982 15 226,734U.S. Government-sponsored mortgage-backed securities398,832 7,601 787 405,646Total held to maturity618,599 14,583 802 632,380Total Investment Securities$1,259,098 $33,062 $1,380 $1,290,780                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
| Debt Securities Credit Loss Recognized in Income, and Other Losses Recorded in Other Comprehensive Income                                       | The following table provides information about debt securities for which only a credit loss was recognized in income and other losses were recorded in other comprehensive income. Accumulated Credit Losses in  Accumulated Credit Losses in  2016 2015Credit losses on debt securities held:   Balance, January 1$— $500   Reductions for previous other-than-temporary losses realized on securities sold during the year—$(500)Balance, December 31$— $—                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| 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 gross unrealized losses and fair value, aggregated by investment category and length of time the individual securities have been in a continuous unrealized loss position at December 31, 2016 and 2015:  Less than 12 Months 12 Months or Longer Total FairValue GrossUnrealizedLosses FairValue GrossUnrealizedLosses FairValue GrossUnrealizedLossesTemporarily Impaired Available for Sale Securities at December 31, 2016           State and municipal$126,593 $5,564   $126,593 $5,564U.S. Government-sponsored mortgage-backed securities185,544 3,071   185,544 3,071Equity securities18,765 1,039     18,765 1,039Total Temporarily Impaired Available for Sale Securities330,902 9,674   330,902 9,674Temporarily Impaired Held to Maturity Securities at December 31, 2016           U.S. Government-sponsored agency securities19,121 479     19,121 479State and municipal50,897 1,796   50,897 1,796U.S. Government-sponsored mortgage-backed securities109,377 1,527   109,377 1,527Total Temporarily Impaired Held to Maturity Securities179,395 3,802   179,395 3,802Total Temporarily Impaired Investment Securities$510,297 $13,476   $510,297 $13,476                         Less than 12 Months 12 Months or Longer Total FairValue GrossUnrealizedLosses FairValue GrossUnrealizedLosses FairValue GrossUnrealizedLossesTemporarily Impaired Available for Sale Securities at December 31, 2015           State and municipal$7,558 $60   $7,558 $60U.S. Government-sponsored mortgage-backed securities83,396 445 $2,101 $73 85,497 518Total Temporarily Impaired Available for Sale Securities90,954 505 2,101 73 93,055 578Temporarily Impaired Held to Maturity Securities at December 31, 2015           State and municipal  1,982 15 1,982 15U.S. Government-sponsored mortgage-backed securities69,641 519 12,906 268 82,547 787Total Temporarily Impaired Held to Maturity Securities69,641 519 14,888 283 84,529 802Total Temporarily Impaired Investment Securities$160,595 $1,024 $16,989 $356 $177,584 $1,380 |
| Amortized Cost and Fair Value of Available for Sale Securities and Held to Maturity Securities by Contractual Maturity                          | 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 Sale Held to Maturity Amortized Cost Fair Value Amortized Cost Fair ValueMaturity Distribution at December 31, 2016       Due in one year or less$2,703 $2,717 $2,046 $2,047Due after one through five years16,359 17,068 61,921 63,193Due after five through ten years60,614 62,241 61,606 61,145Due after ten years281,234 281,763 121,857 121,906 360,910 363,789 247,430 248,291U.S. Government-sponsored mortgage-backed securities313,459 312,292 360,213 363,642Equity securities21,820 20,781  Total Investment Securities$696,189 $696,862 $607,643 $611,933                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |

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| - Definition
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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No definition available.
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| - Definition
Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments.
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| - Definition
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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Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27340-111563
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| - Definition
Tabular disclosure of unrealized gains and losses on investments.
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No definition available.
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