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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share Basic [Abstract]
Net income available to common shareholders - basic	$ 17,896	$ 17,940	$ 58,792	$ 47,389
Basic weighted average common shares	19,121,947	18,968,718	19,092,298	18,911,675
Basic earnings per common share	$ 0.94	$ 0.95	$ 3.08	$ 2.51
Net income available to common shareholders - diluted	$ 17,896	$ 17,940	$ 58,792	$ 47,389
Weighted average common shares	19,121,947	18,968,718	19,092,298	18,911,675
Stock options and performance shares	68,244	127,541	94,100	176,465
Diluted weighted average common shares	19,190,191	19,096,259	19,186,398	19,088,140
Diluted earnings per common share	$ 0.93	$ 0.94	$ 3.06	$ 2.48