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Earnings Per Share - Reconciles Numerator and Denominator of Basic and Diluted Earnings Per Share Computations (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Numerator for EPS:
Net income	$ 20,972us-gaap_NetIncomeLoss	$ 21,341us-gaap_NetIncomeLoss	$ 17,126us-gaap_NetIncomeLoss	$ 13,549us-gaap_NetIncomeLoss	$ 15,582us-gaap_NetIncomeLoss	$ 14,407us-gaap_NetIncomeLoss	$ 17,029us-gaap_NetIncomeLoss	$ 11,959us-gaap_NetIncomeLoss	$ 72,988us-gaap_NetIncomeLoss	$ 58,977us-gaap_NetIncomeLoss	$ 30,312us-gaap_NetIncomeLoss
Less: Preferred stock dividends									(13)	(29)	(287)
Income available to common stockholders									$ 72,975	$ 58,948	$ 30,025
Denominator for EPS:
Weighted average common shares outstanding									33,569	35,866	39,614
Plus: Assumed conversion of stock-based compensation									1,535	1,531	513
Assumed conversion of preferred stock									46	379	489
Weighted average diluted common shares outstanding									35,150	37,776	40,616
Basic earnings per common share	$ 0.63us-gaap_EarningsPerShareBasic	$ 0.64us-gaap_EarningsPerShareBasic	$ 0.50us-gaap_EarningsPerShareBasic	$ 0.41us-gaap_EarningsPerShareBasic	$ 0.46us-gaap_EarningsPerShareBasic	$ 0.43us-gaap_EarningsPerShareBasic	$ 0.47us-gaap_EarningsPerShareBasic	$ 0.30us-gaap_EarningsPerShareBasic	$ 2.17us-gaap_EarningsPerShareBasic	$ 1.64us-gaap_EarningsPerShareBasic	$ 0.76us-gaap_EarningsPerShareBasic
Diluted earnings per common share	$ 0.59us-gaap_EarningsPerShareDiluted	$ 0.61us-gaap_EarningsPerShareDiluted	$ 0.49us-gaap_EarningsPerShareDiluted	$ 0.38us-gaap_EarningsPerShareDiluted	$ 0.44us-gaap_EarningsPerShareDiluted	$ 0.40us-gaap_EarningsPerShareDiluted	$ 0.44us-gaap_EarningsPerShareDiluted	$ 0.29us-gaap_EarningsPerShareDiluted	$ 2.08us-gaap_EarningsPerShareDiluted	$ 1.56us-gaap_EarningsPerShareDiluted	$ 0.75us-gaap_EarningsPerShareDiluted
Weighted average number of antidilutive shares									64	518	818