Earnings Per Common Share - Summary of Computation of Basic and Diluted Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income available to common shareholders	$ 84,090	$ 77,227	$ 75,205
Denominator:
Weighted-average shares	15,394,971	15,412,365	15,407,078
Effect of dilutive securities – restricted stock units and warrants	18,861	0	1,063
Earnings per common share:	15,413,832	15,412,365	15,408,141
Earnings per common share:
Basic earnings per common share	$ 5.46us-gaap_EarningsPerShareBasic	$ 5.01us-gaap_EarningsPerShareBasic	$ 4.88us-gaap_EarningsPerShareBasic
Diluted earnings per common share	$ 5.46us-gaap_EarningsPerShareDiluted	$ 5.01us-gaap_EarningsPerShareDiluted	$ 4.88us-gaap_EarningsPerShareDiluted

The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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Net income after adjustments for dividends on preferred stock (declared in the period) and/or cumulative preferred stock (accumulated for the period).

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The sum of dilutive potential common shares or units used in the calculation of the diluted per-share or per-unit computation.

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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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