Net Income Per Common Share - Reconciliation of Numerator and Denominator of Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 55,054	$ 75,036	$ 85,377
Less: non-forfeitable dividends on restricted stock	10	12	9
Net income available to common shareholders	$ 55,044	$ 75,024	$ 85,368
Weighted average common shares outstanding (in shares)	72,479,807	74,132,863	77,066,459
Weighted average potential dilutive common shares (in shares)	392,381	340,506	114,664
Weighted average common shares outstanding-assuming dilution (in shares)	72,872,188	74,473,369	77,181,123
Net income per common share - basic (in usd per share)	$ 0.76	$ 1.01	$ 1.11
Net income per common share - diluted (in usd per share)	$ 0.76	$ 1.01	$ 1.11