Earnings Per Share - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Earnings Per Share [Abstract]
Net income available to common shareholders	$ 15,626	$ 15,255
Less: earnings allocated to participating securities	141	141
Net income allocated to common shareholders	$ 15,485	$ 15,114
Weighted average number of common shares outstanding - basic (in shares)	50,275	47,503
Share-based plans (in shares)	167	233
Weighted average number of common shares outstanding - diluted (in shares)	50,442	47,736
Basic earnings per share (in usd per share)	$ 0.31	$ 0.32
Diluted earnings per share (in usd per share)	$ 0.31	$ 0.32