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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share Basic [Abstract]
Net income	$ 19,701	$ 16,577	$ 52,826	$ 45,582
Basic weighted average common shares	19,216,545	19,171,912	19,204,196	19,161,529
Basic earnings per common share	$ 1.03	$ 0.86	$ 2.75	$ 2.38
Net income	$ 19,701	$ 16,577	$ 52,826	$ 45,582
Weighted average common shares	19,216,545	19,171,912	19,204,196	19,161,529
Stock options and restricted stock	56,439	64,202	76,840	74,426
Diluted weighted average common shares	19,272,984	19,236,114	19,281,036	19,235,955
Diluted earnings per common share	$ 1.02	$ 0.86	$ 2.74	$ 2.37