| **Share-Based Compensation (Tables)**                                         | 6 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: Three Months EndedJune 30, Six Months EndedJune 30,  2016 2015 2016 2015Stock and ESPP Options       Pre-tax compensation expense$21 $28 $36 $54Income tax expense (benefit)10  10 (1)Stock and ESPP option expense, net of income taxes$31 $28 $46 $53Restricted Stock Awards       Pre-tax compensation expense$620 $586 $1,199 $1,076Income tax benefit(217) (198) (420) (366)Restricted stock awards expense, net of income taxes$403 $388 $779 $710Total Share-Based Compensation       Pre-tax compensation expense$641 $614 $1,235 $1,130Income tax benefit(207) (198) (410) (367)Total share-based compensation expense, net of income taxes$434 $416 $825 $763 |
| Stock option activity under stock option plans                                | Stock option activity under the Corporation's stock option plans as of June 30, 2016 and changes during the six months ended June 30, 2016, were as follows: Number ofSharesWeighted-Average Exercise PriceWeighted Average RemainingContractual Term(in Years)AggregateIntrinsicValueOutstanding at January 1, 2016442,012$19.99  Granted—  Exercised(13,828)$15.94  Canceled(49,482)$25.14  Outstanding June 30, 2016378,702$19.473.052,313,252Vested and Expected to Vest at June 30, 2016378,702$19.473.022,313,252Exercisable at June 30, 2016378,702$19.473.052,313,252                                                                                                                                                                                                                                                                                                                                                                                                                                    |
| Unvested RSAs outstanding                                                     | The following table summarizes information on unvested RSAs outstanding as of June 30, 2016: Number of SharesWeighted-AverageGrant Date Fair ValueUnvested RSAs at January 1, 2016354,504$19.65Granted116,133$23.48Vested(106,290)$15.39Forfeited(4,943)$22.28Unvested RSAs at June 30, 2016359,404$22.42                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |

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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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