Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share Basic [Abstract]
Net income available to common shareholders - basic	$ 16,577	$ 17,896	$ 45,582	$ 58,792
Basic weighted average common shares	19,171,912	19,121,947	19,161,529	19,092,298
Basic earnings per common share	$ 0.86	$ 0.94	$ 2.38	$ 3.08
Net income available to common shareholders - diluted	$ 16,577	$ 17,896	$ 45,582	$ 58,792
Weighted average common shares	19,171,912	19,121,947	19,161,529	19,092,298
Stock options and restricted stock	64,202	68,244	74,426	94,100
Diluted weighted average common shares	19,236,114	19,190,191	19,235,955	19,186,398
Diluted earnings per common share	$ 0.86	$ 0.93	$ 2.37	$ 3.06