| **Share-Based Compensation (Tables)**                                         | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| **Disclosure of Compensation Related Costs, Share-based Payments [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
| Components of Share Based Compensation Awards                                 | The following table summarizes the components of the Corporation's share-based compensation awards recorded as expense: Years Ended December 31,  2016 2015 2014Stock and ESPP Options     Pre-tax compensation expense$76 $101 $237Income tax expense (benefit)18 4 (47)Stock and ESPP option expense, net of income taxes$94 $105 $190Restricted Stock Awards     Pre-tax compensation expense$2,524 $2,169 $1,940Income tax benefit(883) (749) (679)Restricted stock awards expense, net of income taxes$1,641 $1,420 $1,261Total Share-Based Compensation:     Pre-tax compensation expense$2,600 $2,270 $2,177Income tax benefit(865) (745) (726)Total share-based compensation expense, net of income taxes$1,735 $1,525 $1,451 |
| Stock Option Activity Under Stock Option Plans                                | Stock option activity under the Corporation's stock option plans, as of December 31, 2016, and changes during the year ended December 31, 2016, were as follows:  Number ofShares Weighted-AverageExercise Price Weighted AverageRemaining ContractualTerm (in Years) AggregateIntrinsic ValueOutstanding at January 1, 2016442,012 $19.99    Exercised(128,848) $19.37  Canceled(52,953) $25.09    Outstanding December 31, 2016260,211 $19.26 2.71 $4,785,982Vested and Expected to Vest at December 31, 2016260,211 $19.26 2.71 $4,785,982Exercisable at December 31, 2016260,211 $19.26 2.71 $4,785,982                                                                                                                                                                        |
| Summary of Unvested RSAs Outstanding                                          | The following table summarizes information on unvested RSAs outstanding as of December 31, 2016: Number ofShares Weighted-AverageGrant Date Fair ValueUnvested RSAs at January 1, 2016354,504 $19.65Granted124,563 $24.01Forfeited(9,594) $21.99Vested(141,126) $16.62Unvested RSAs at December 31, 2016328,347 $22.87                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |

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| - Definition
Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed.
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| - Definition
Tabular disclosure of the changes in outstanding nonvested restricted stock units.
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| - Definition
Tabular disclosure of the number and weighted-average exercise prices (or conversion ratios) for share options (or share units) that were outstanding at the beginning and end of the year, vested and expected to vest, exercisable or convertible at the end of the year, and the number of share options or share units that were granted, exercised or converted, forfeited, and expired during the year.
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