Earnings Per Common Share - Summary of Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Numerator for both basic and diluted earnings per common share:
Net income	$ 33,169	$ 26,341	$ 66,698	$ 52,228
Denominator:
Total average basic common shares outstanding	46,498,305	43,995,749	45,281,264	43,971,789
Effect of dilutive stock options and other stock compensation	141,475	65,672	135,746	75,023
Total diluted average common shares outstanding	46,639,780	44,061,421	45,417,010	44,046,812
Earnings per common share - basic	$ 0.71	$ 0.60	$ 1.47	$ 1.19
Earnings per common share - diluted	$ 0.71	$ 0.60	$ 1.47	$ 1.19