Stockholders' Equity - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Numerator:
Net income available to common shareholders	$ 37,110	$ 27,949
Denominator:
Basic weighted average shares outstanding	29,427,578	29,363,210
Effect of dilutive securities:
Diluted weighted average shares outstanding	29,599,811	29,561,406
Basic earnings per share	$ 1.26	$ 0.95
Diluted earnings per share	$ 1.25	$ 0.95
Stock Appreciation Rights and Options [Member]
Effect of dilutive securities:
Dilutive securities	97,028	100,585
Restricted Stock [Member]
Effect of dilutive securities:
Dilutive securities	75,205	97,611

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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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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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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