Stockholders' Equity - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income available to common shareholders	$ 18,715	$ 15,161	$ 36,268	$ 27,456
Denominator:
Basic weighted average shares outstanding	31,735,333	32,898,288	31,709,645	33,012,845
Effect of dilutive securities:
Stock appreciation rights and options	226,680	263,532	233,373	283,223
Restricted stock	180,926	145,643	176,008	156,746
Diluted weighted average shares outstanding	32,142,939	33,307,463	32,119,026	33,452,814
Basic earnings per share	$ 0.59	$ 0.46	$ 1.14	$ 0.83
Diluted earnings per share	$ 0.58	$ 0.46	$ 1.13	$ 0.82