Net Income Per Common Share - Basic and Diluted Net Income Per Common Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Earnings Per Share Reconciliation [Abstract]
Income available to common stockholders	$ 16,259	$ 14,657
Income available to common stockholders plus assumed exercises of stock options	$ 16,259	$ 14,657
Income available to common stockholders - Shares	15,507,346	15,342,486
Effect of stock options - Shares	331,202	318,434
Income available to common stockholders plus assumed exercises of stock options - Shares	15,838,548	15,660,920
Income available to common stockholders - Per Share Amount	$ 1.05us-gaap_EarningsPerShareBasic	$ 0.96us-gaap_EarningsPerShareBasic
Income available to common stockholders plus assumed exercises of stock options - Per Share Amount	$ 1.03us-gaap_EarningsPerShareDiluted	$ 0.94us-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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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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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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The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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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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