Earnings Per Share - Schedule of Calculation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings per common share - basic
Net income	$ 25,126	$ 22,336
Net income allocated to common shareholders	24,802	21,933
Weighted average common shares outstanding	33,168	29,270
Earnings per common share - Basic	$ 0.75us-gaap_EarningsPerShareBasic	$ 0.75us-gaap_EarningsPerShareBasic
Earnings per common share - diluted
Net income allocated to common shareholders	24,802	21,933
Net income allocated to common shareholders	24,782	21,931
Weighted average common shares outstanding	33,168	29,270
Dilutive potential common shares - stock options	67	147
Weighted average common shares outstanding - diluted	33,235	29,417
Earnings per common share - diluted	$ 0.75us-gaap_EarningsPerShareDiluted	$ 0.75us-gaap_EarningsPerShareDiluted
Unvested Restricted Stock [Member]
Earnings per common share - basic
Dividends and undistributed earnings allocated to unvested restricted shares	(324)	(403)
Earnings per common share - diluted
Dividends and undistributed earnings allocated to unvested restricted shares	$ (20)	$ (2)

Dilutive potential common shares stock options.

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Distributed and undistributed earnings reallocated to unvested restricted stock.

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The total amount of net income in the current period allocated to common shareholders under the two-class method of calculating earnings per share on a basic per share basis.

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Distributed and undistributed earnings to unvested restricted stock basic.

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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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Net Income or Loss Available to Common Stockholders plus adjustments resulting from the assumption that dilutive convertible securities were converted, options or warrants were exercised, or that other shares were issued upon the satisfaction of certain conditions.

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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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