Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share [Abstract]
Net income available to common shareholders	$ 15,838	$ 13,370	$ 31,093	$ 25,682
Less: earnings allocated to participating securities	146	119	287	239
Net income allocated to common shareholders	$ 15,692	$ 13,251	$ 30,806	$ 25,443
Weighted average number of common shares outstanding - basic (in shares)	47,600	47,465	47,552	47,410
Share-based plans (in shares)	170	209	201	236
Weighted average number of common shares outstanding - diluted (in shares)	47,770	47,674	47,753	47,646
Basic earnings per share (in usd per share)	$ 0.33	$ 0.28	$ 0.65	$ 0.54
Diluted earnings per share (in usd per share)	$ 0.33	$ 0.28	$ 0.65	$ 0.53