Quarterly Financial Data - Summary of Consolidated Quarterly Financial Data (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended												12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Selected Quarterly Financial Information [Line Items]
Interest income	$ 162,969	$ 167,230	$ 166,858	$ 174,136	$ 166,572		$ 171,639		$ 167,794		$ 167,819		$ 671,193	$ 673,824	$ 676,812
Interest expense	44,440	45,408	44,787	46,677	48,258		49,704		50,124		50,927		181,312	199,013	214,060
Net interest income	118,529	121,822	122,071	127,459	118,314		121,935		117,670		116,892		489,881	474,811	462,752
Provision for credit losses	5,200	7,250	7,405	5,697	15,364		7,783		6,026		24,162		25,552	53,335	49,456
(Losses) gains on securities transactions, net	44	1,496	1,204	(157)	12,034		863		16,492		2,679		2,587	32,068	11,598
Net impairment losses on securities recognized in earnings		(4,697)	(550)		(19,143)						(825)		5,247	19,968	4,642
Trading (losses) gains, net	2,166	6	1,609	(988)	(839)		776		(1,048)		3,382		2,793	2,271	(6,897)
Gains on sales of loans, net	15,636	25,055	3,141	3,166									46,998	10,699	12,591
Other non-interest income	15,979	18,636	18,626	20,574	21,720		18,564		18,091		39,551		21,969	15,921	16,015
Non-interest expense	95,623	93,219	91,510	94,548	84,869	[1]	85,794	[1]	83,572	[1]	84,321	[1]	374,900	338,556	319,688
Income before income taxes	51,531	61,849	47,186	49,809	31,853	[1]	48,561	[1]	61,607	[1]	53,196	[1]	210,375	195,217	184,935
Income tax expense (benefit)	14,702	22,402	14,366	15,278	7,321	[1]	13,489	[1]	24,999	[1]	16,897	[1]	66,748	62,706	54,929
Net income	$ 36,829	$ 39,447	$ 32,820	$ 34,531	$ 24,532	[1]	$ 35,072	[1]	$ 36,608	[1]	$ 36,299	[1]	$ 143,627	$ 132,511	$ 130,006
Earnings per common share, Basic	$ 0.19	$ 0.20	$ 0.17	$ 0.18	$ 0.14		$ 0.20		$ 0.21		$ 0.20		$ 0.73	$ 0.74	$ 0.73
Earnings per common share, Diluted	$ 0.19	$ 0.20	$ 0.17	$ 0.18	$ 0.14		$ 0.20		$ 0.21		$ 0.20		$ 0.73	$ 0.74	$ 0.73
Cash dividends declared per common share	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.16		$ 0.16		$ 0.16		$ 0.16		$ 0.65	$ 0.66	$ 0.66
Average common shares outstanding, Basic	197,795,817	197,437,988	197,246,322	196,930,733	178,694,711		178,507,769		178,335,522		178,154,684		197,354,159	178,424,883	177,568,546
Average common shares outstanding, Diluted	197,795,817	197,437,988	197,250,168	196,961,915	178,695,174		178,508,382		178,345,558		178,162,788		197,354,372	178,426,070	177,577,663

[1] Previously reported amounts have been revised to reflect an increase in non-interest expense due to the immaterial correction of an error. See the "Correction of an Immaterial Error" section of Note 1 to the consolidated financial statements for additional information.