Earnings Per Share - Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended			6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share [Abstract]
Net income per consolidated statements of income	$ 4,125	$ 42,404	$ 42,404	$ 7,811	$ 44,498
Net income allocated to participating securities	(25)		(235)	(39)	(232)
Net income allocated to common stock	4,100		42,169	7,772	44,266
Distributed earnings allocated to common stock	490			979
Undistributed earnings allocated to common stock	3,610		42,169	6,793	44,266
Net income allocated to common stock	$ 4,100		$ 42,169	$ 7,772	$ 44,266
Weighted average common shares outstanding, including shares considered participating securities	48,649		50,554	48,968	50,481
Less: Average participating securities	(290)		(280)	(244)	(263)
Weighted average shares	48,359		50,274	48,724	50,218
Basic earnings per common share	$ 0.08	$ 0.84	$ 0.84	$ 0.16	$ 0.88
Weighted average common shares outstanding for basic earnings per common share	48,359		50,274	48,724	50,218
Add: Dilutive effects of assumed exercises of stock options	275		222	273	221
Weighted average shares and dilutive potential common shares	48,634		50,496	48,997	50,439
Diluted earnings per common share	$ 0.08	$ 0.84	$ 0.84	$ 0.16	$ 0.88