Quarterly Financial Data - Summary of Quarterly Financial Data (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 170,942	$ 155,459	$ 157,164	$ 153,038	$ 156,915	$ 153,779	$ 152,370	$ 153,033	$ 636,603	$ 616,097	$ 671,193
Interest expense	42,296	40,791	39,745	39,014	40,787	42,110	42,483	42,997	161,846	168,377	181,312
Net interest income	128,646	114,668	117,419	114,024	116,128	111,669	109,887	110,036	474,757	447,720	489,881
Provision for credit losses	3,980	(423)	(5,671)	3,998	6,440	5,334	2,552	1,769	1,884	16,095	25,552
Non-Interest Income
(Losses) gains on securities transactions, net	643	103	7	(8)	10,670	9	41	3,958	745	14,678	2,587
Trading (losses) gains, net					1,150	2,231	(270)	(2,202)	0	0	(5,247)
Trading (losses) gains, net	47	(35)	(34)	(9)	1,540	2,729	14,366	15,060	(31)	909	2,793
Gains on sales of loans, net	234	(95)	679	913	11,547	(1,010)	678	(268)	1,731	33,695	46,998
Gains (losses) on sales of assets, net	17,876	83	276	(148)					18,087	10,947	(329)
Other non-interest income	10,763	14,725	11,606	19,990	17,166	18,431	18,079	14,748	15,335	15,237	21,969
Non-Interest Expense
Non-interest expense:					88,178	92,434	92,718	93,656	403,255	381,338	374,900
Loss on extinguishment of debt	10,132	0	0	0					10,132	0	0
Amortization of tax credit investments	10,048	4,630	5,802	3,716	7,914	2,027	2,628	1,783	24,196	14,352	4,157
Other	101,087	86,906	88,551	92,383					55,451	53,299	53,347
Income before income taxes	32,962	38,336	41,271	34,665	55,669	34,264	44,883	44,124	147,234	178,940	210,375
Income tax expense	7,827	10,654	11,751	830	16,061	7,143	10,961	12,814	31,062	46,979	66,748
Net Income	$ 25,135	$ 27,682	$ 29,520	$ 33,835	$ 39,608	$ 27,121	$ 33,922	$ 31,310	$ 116,172	$ 131,961	$ 143,627
Earnings Per Common Share:
Basic (in usd per share)	$ 0.11	$ 0.14	$ 0.15	$ 0.17	$ 0.20	$ 0.14	$ 0.17	$ 0.16	$ 0.56	$ 0.66	$ 0.73
Diluted (in usd per share)	$ 0.11	$ 0.14	$ 0.15	$ 0.17	$ 0.20	$ 0.14	$ 0.17	$ 0.16	$ 0.56	$ 0.66	$ 0.73
Cash Dividends Declared Per Common Share (in usd per share)	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.16	$ 0.16	$ 0.16	$ 0.444	$ 0.60	$ 0.65
Weighted Average Number of Common Shares Outstanding:
Basic (in shares)	221,471,635	200,614,091	200,472,592	200,128,384	199,613,524	199,445,874	199,244,243	198,924,995	205,716,293	199,309,425	197,354,159
Diluted (in shares)	221,471,635	200,614,091	200,472,592	200,128,384	199,613,524	199,445,874	199,244,243	198,924,995	205,716,293	199,309,425	197,354,372