| **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,  20142013 2014 2013Stock and ESPP Options     Pre-tax compensation expense$30$50 $74$88Income tax expense (benefit)(2)20 (5)18Stock and ESPP option expense, net of income taxes$28$70 $69$106Restricted Stock Awards     Pre-tax compensation expense$524$386 $985$722Income tax benefit(183)(135) (344)(252)Restricted stock awards expense, net of income taxes$341$251 $641$470Total Share-Based Compensation     Pre-tax compensation expense$554$436 $1,059$810Income tax benefit(185)(115) (349)(234)Total share-based compensation expense, net of income taxes$369$321 $710$576 |
| Stock Option Activity Under Stock Option Plans                                | Stock option activity under the Corporation's stock option plans as of June 30, 2014 and changes during the six months ended June 30, 2014, were as follows: Number ofSharesWeighted-Average Exercise PriceWeighted Average RemainingContractual Term(in Years)AggregateIntrinsicValueOutstanding at January 1, 2014958,786$21.32  Granted  Exercised(38,650)$11.64  Canceled(68,707)$22.55  Outstanding June 30, 2014851,429$21.653.462,149,732Vested and Expected to Vest at June 30, 2014851,429$21.652.952,149,732Exercisable at June 30, 2014842,429$21.723.392,097,352                                                                                                                                                                                                                                                                                  |
| Unvested RSAs Outstanding                                                     | The following table summarizes information on unvested RSAs outstanding as of June 30, 2014: Number of SharesWeighted-AverageGrant Date Fair ValueUnvested RSAs at January 1, 2014429,002$12.51Granted89,946$20.47Vested(3,856)$16.68Forfeited(117,185)$9.30Unvested RSAs at June 30, 2014397,907$15.24                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |

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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.
+ References
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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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