Earnings Per Share - Schedule of Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Earnings per common share - basic
Net income	$ 42,475	$ 30,893	$ 98,437	$ 69,445
Dividends and undistributed earnings allocated to unvested restricted shares	(492)	(462)	(1,171)	(1,114)
Earnings Allocated to Common Shareholders	$ 41,983	$ 30,431	$ 97,266	$ 68,331
Weighted average common shares outstanding (in shares)	40,514,000	32,803,000	37,436,000	30,791,000
Earnings per common share - Basic (in usd per share)	$ 1.04	$ 0.93	$ 2.60	$ 2.22
Earnings per common share - diluted
Net income allocated to common shareholders - basic	$ 41,983	$ 30,431	$ 97,266	$ 68,331
Dividends and undistributed earnings allocated to unvested restricted shares	(3)	(4)	(41)	(24)
Net income allocated to common shareholders - diluted	$ 41,980	$ 30,427	$ 97,225	$ 68,307
Weighted average common shares outstanding (in shares)	40,514,000	32,803,000	37,436,000	30,791,000
Dilutive potential common shares - stock options (in shares)	100,000	124,000	96,000	132,000
Weighted average common shares outstanding - diluted (in shares)	40,614,000	32,927,000	37,532,000	30,923,000
Earnings per common share - Diluted (in usd per share)	$ 1.03	$ 0.92	$ 2.59	$ 2.21
Weighted average number of shares owned by Recognition and Retention Plan	616,307	621,341	606,621	630,320
Stock options having antidilutive effect on earnings per share	82,310	12,765	79,690	12,765