Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basic earnings per share
Net income available to common shareholders, Income	$ 52,580	$ 41,518	$ 32,481
Less: earnings allocated to participating securities, Income	(480)	(396)	(263)
Net income available to common shareholders, Total	$ 52,100	$ 41,122	$ 32,218
Net income available to common shareholders, Shares	47,438	42,912	37,844
Less: earnings allocated to participating securities, Shares	0	0	0
Net income available to common shareholders, Total	47,438	42,912	37,844
Net income available to common shareholders, Per share amount	$ 1.11	$ 0.97	$ 0.86
Less: earnings allocated to participating securities, Per share amount	(0.01)	(0.01)	(0.01)
Net income available to common shareholders, Total	$ 1.10	$ 0.96	$ 0.85
Effect of dilutive securities
Stock options and restricted stock, Income	$ 0	$ 0	$ 0
Stock options and restricted stock, Shares	236	202	149
Stock options and restricted stock, Per share amount	$ (0.01)	$ (0.01)	$ 0.00
Diluted earnings per share
Net income available to common shareholders plus assumed conversions, Income	$ 52,100	$ 41,122	$ 32,218
Net income available to common shareholders plus assumed conversions, Shares	47,674	43,114	37,993
Net income available to common shareholders plus assumed conversions, Per share amount	$ 1.09	$ 0.95	$ 0.85