Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share Basic [Abstract]
Net income available to common shareholders – basic	$ 649	$ 16,577	$ 15,481	$ 13,524	$ 19,073	$ 17,896	$ 16,639	$ 24,257	$ 46,231	$ 77,865	$ 70,462
Basic weighted-average common shares									19,165,489	19,105,806	18,941,077
Basic earnings per share	$ 0.03	$ 0.86	$ 0.81	$ 0.71	$ 1.00	$ 0.94	$ 0.87	$ 1.27	$ 2.41	$ 4.08	$ 3.72
Net income available to common shareholders - diluted									$ 46,231	$ 77,865	$ 70,462
Weighted average common shares									19,165,489	19,105,806	18,941,077
Stock options and restricted stock									80,377	97,844	178,109
Diluted weighted average common shares									19,245,866	19,203,650	19,119,186
Diluted earnings per common share	$ 0.03	$ 0.86	$ 0.81	$ 0.70	$ 0.99	$ 0.93	$ 0.87	$ 1.27	$ 2.40	$ 4.05	$ 3.69