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, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Earnings Per Share Basic [Abstract]
Net income available to common shareholders – basic									$ 77,865	$ 70,462	$ 53,666
Basic weighted-average common shares									19,105,806	18,941,077	18,646,128
Basic earnings per share	$ 1.00	$ 0.94	$ 0.87	$ 1.27	$ 1.21	$ 0.95	$ 0.76	$ 0.80	$ 4.08	$ 3.72	$ 2.88
Net income available to common shareholders - diluted	$ 19,073	$ 17,896	$ 16,639	$ 24,257	$ 23,073	$ 17,940	$ 14,319	$ 15,130	$ 77,865	$ 70,462	$ 53,666
Weighted average common shares									19,105,806	18,941,077	18,646,128
Stock options and performance shares									97,844	178,109	282,376
Diluted weighted average common shares									19,203,650	19,119,186	18,928,504
Diluted earnings per common share	$ 0.99	$ 0.93	$ 0.87	$ 1.27	$ 1.21	$ 0.94	$ 0.75	$ 0.79	$ 4.05	$ 3.69	$ 2.84