Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Earnings Per Share [Abstract]
Net income available to common shareholders	$ 16,758	$ 13,723	$ 47,851	$ 39,405
Less: earnings allocated to participating securities	153	122	440	362
Net income allocated to common shareholders	$ 16,605	$ 13,601	$ 47,411	$ 39,043
Weighted average number of common shares outstanding - basic (in shares)	47,605	47,466	47,570	47,429
Share-based plans (in shares)	183	226	194	231
Weighted average number of common shares outstanding - diluted (in shares)	47,788	47,692	47,764	47,660
Basic earnings per share (in usd per share)	$ 0.35	$ 0.29	$ 1.00	$ 0.82
Diluted earnings per share (in usd per share)	$ 0.35	$ 0.29	$ 0.99	$ 0.82