Net Income Per Common Share - Schedule of Reconciliation of Numerator and Denominator of Income Per Common Share (Detail) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income		$ 20,267	$ 17,773	$ 39,330	$ 35,979
Less: non-forfeitable dividends on restricted stock		3		3
Net income available to common shareholders		$ 20,264	$ 17,773	$ 39,327	$ 35,979
Weighted average common shares outstanding		77,457,322	78,410,554	77,631,339	78,393,042
Weighted average potential dilutive common shares	[1]	240,539	789,410	148,863	811,600
Weighted average common shares outstanding - assuming dilution		77,697,861	79,199,964	77,780,202	79,204,642
Net income per common share - basic		$ 0.26	$ 0.23	$ 0.51	$ 0.46
Net income per common share - diluted		$ 0.26	$ 0.22	$ 0.51	$ 0.45

[1] Potential common shares consist of common stock issuable under the assumed exercise of stock options and restricted stock awards using the treasury stock method.