Quarterly Financial Data (unaudited) - Unaudited Condensed Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operations Data:
Net interest income	$ 17,015	$ 17,286	$ 17,424	$ 17,056	$ 17,501	$ 17,558	$ 17,862	$ 18,315	$ 48,106	$ 68,781	$ 71,236	$ 62,926
Provision for loan losses	(5,086)	(22,410)	(646)	(1,354)	(3,281)	(1,304)	0	0	275	(29,496)	(4,585)	950
Non-interest income	7,772	8,721	9,738	8,740	8,962	8,015	7,610	5,932	159,533	34,971	30,519	167,036
Non-interest expense	18,149	22,552	23,263	20,973	24,621	21,915	22,855	22,317	95,732	84,937	91,708	131,400
Income (loss) before income tax expense	11,724	25,865	4,545	6,177	5,123	4,962	2,617	1,930	111,632	48,311	14,632	97,612
Income tax expense	3,103	10,394	(102,976)	32	0	0	10	0	9	(89,447)	10	9
Net income (loss)	$ 8,621	$ 15,471	$ 107,521	$ 6,145	$ 5,123	$ 4,962	$ 2,607	$ 1,930	$ 111,623	$ 137,758	$ 14,622	$ 97,603
Per Share:
Basic income per common share (usd per share)	$ 0.91	$ 1.64	$ 11.42	$ 0.66	$ 0.56	$ 0.55	$ 0.29	$ 0.21	$ 12.18	$ 14.64	$ 1.61
Diluted income per common share (usd per share)	0.91	1.62	11.37	0.65	0.55	0.55	0.29	0.21	12.18	14.57	1.60
Dividends per common share (usd per share)	0	0	0	0	0	0	0	0	$ 0	$ 0	$ 0
Book value per common share (usd per share)	$ 38.20	$ 37.49	$ 35.68	$ 24.66	$ 23.85	$ 23.22	$ 23.37	$ 22.84