Loans - Weighted Average Risk Grades for All Commercial Loans by Class (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2013 Grade	Mar. 31, 2012 Grade	Dec. 31, 2012 Grade
Accounts, Notes, Loans and Financing Receivable [Line Items]
Loans	$ 4,708,665	$ 4,042,852	$ 4,828,612
Loans	3,391,509	3,034,884	3,319,795
Loans	647,862	493,600	608,306
Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	5.31	5.38	5.25
Loans	1,070,596	994,979	1,083,186
Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.21	6.55	6.21
Loans	3,638,069	3,047,873	3,745,426
Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.95	7.12	6.97
Loans	2,743,647	2,541,284	2,711,489
Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.92	7.19	6.94
Loans	647,862	493,600	608,306
Risk Grades 1-8 [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	5.30	5.37	5.24
Loans	1,067,466	992,463	1,081,725
Risk Grades 1-8 [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	5.89	6.20	5.81
Loans	3,367,114	2,803,976	3,367,443
Risk Grades 1-8 [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.63	6.67	6.63
Loans	2,502,845	2,233,155	2,460,448
Risk Grades 1-8 [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	6.81	6.97	6.82
Loans	618,852	454,674	579,108
Risk Grade 9 [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00	9.00
Loans	2,745	2,516	392
Risk Grade 9 [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00	9.00
Loans	122,482	65,376	250,508
Risk Grade 9 [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00	9.00
Loans	86,783	104,727	92,041
Risk Grade 9 [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	9.00	9.00	9.00
Loans	24,378	16,062	23,046
Risk Grade 10 [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00	10.00
Loans
Risk Grade 10 [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00	10.00
Loans	33,818	35,504	28,440
Risk Grade 10 [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00	10.00
Loans	34,120	33,641	42,603
Risk Grade 10 [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	10.00	10.00	10.00
Loans	2,968	15,442	4,435
Risk Grade 11 [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00	11.00
Loans	385
Risk Grade 11 [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00	11.00
Loans	66,400	93,415	53,797
Risk Grade 11 [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00	11.00
Loans	80,729	127,869	77,658
Risk Grade 11 [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	11.00	11.00	11.00
Loans	609	6,137	617
Risk Grade 12 [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00	12.00
Loans			169
Risk Grade 12 [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00	12.00
Loans	42,230	44,520	40,603
Risk Grade 12 [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00	12.00
Loans	35,880	40,486	35,602
Risk Grade 12 [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	12.00	12.00	12.00
Loans	1,055	1,285	1,100
Risk Grade 13 [Member] | Commercial and Industrial, Energy [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00	13.00
Loans			900
Risk Grade 13 [Member] | Commercial and Industrial, Other Commercial [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00	13.00
Loans	6,025	5,082	4,635
Risk Grade 13 [Member] | Commercial Real Estate, Buildings, Land and Other [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00	13.00
Loans	3,290	1,406	3,137
Risk Grade 13 [Member] | Commercial Real Estate, Construction [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Risk Grade	13.00	13.00	13.00
Loans