Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 57,452	$ 61,676	$ 67,187	$ 61,234
Charge-offs	(5,583)	(4,670)	(32,195)	(13,411)
Recoveries	374	354	1,954	1,453
Provision for Loan, Lease, and Other Losses	2,714	6,754	18,011	14,838
Ending Balance	54,957	64,114	54,957	64,114
Ending balance: individually evaluated for impaired	8,239	12,914	8,239	12,914
Ending balance: collectively evaluated for impaired	46,718	51,200	46,718	51,200
Total	4,240,004	4,214,299	4,240,004	4,214,299	4,204,704
Ending balance: individually evaluated for impaired	64,639	88,853	64,639	88,853
Ending balance: collectively evaluated for impaired	4,175,365	4,125,446	4,175,365	4,125,446
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	16,575	19,302	19,852	18,200
Charge-offs	(3,081)	(1,271)	(17,302)	(4,939)
Recoveries	81	74	345	349
Provision for Loan, Lease, and Other Losses	1,358	(25)	12,038	4,470
Ending Balance	14,933	18,080	14,933	18,080
Ending balance: individually evaluated for impaired	4,889	7,306	4,889	7,306
Ending balance: collectively evaluated for impaired	10,044	10,774	10,044	10,774
Total	1,008,316	1,087,019	1,008,316	1,087,019	1,019,822
Ending balance: individually evaluated for impaired	25,620	32,833	25,620	32,833
Ending balance: collectively evaluated for impaired	982,696	1,054,186	982,696	1,054,186
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	8,246	8,001	8,928	6,756
Charge-offs	(18)	(2,016)	(773)	(2,356)
Recoveries	81	29	140	121
Provision for Loan, Lease, and Other Losses	(1,879)	2,848	(1,865)	4,341
Ending Balance	6,430	8,862	6,430	8,862
Ending balance: individually evaluated for impaired	165	973	165	973
Ending balance: collectively evaluated for impaired	6,265	7,889	6,265	7,889
Total	74,025	95,425	74,025	95,425	87,438
Ending balance: individually evaluated for impaired	4,028	11,427	4,028	11,427
Ending balance: collectively evaluated for impaired	69,997	83,998	69,997	83,998
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6,439	6,619	5,908	8,237
Charge-offs	(757)	(530)	(1,400)	(2,984)
Recoveries	71	49	883	331
Provision for Loan, Lease, and Other Losses	84	566	446	1,120
Ending Balance	5,837	6,704	5,837	6,704
Ending balance: individually evaluated for impaired	1,377	719	1,377	719
Ending balance: collectively evaluated for impaired	4,460	5,985	4,460	5,985
Total	1,280,990	1,228,328	1,280,990	1,228,328	1,241,565
Ending balance: individually evaluated for impaired	9,794	7,224	9,794	7,224
Ending balance: collectively evaluated for impaired	1,271,196	1,221,104	1,271,196	1,221,104
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	16,128	17,638	22,441	18,961
Charge-offs	(813)	(97)	(10,051)	(638)
Recoveries	13	70	121	256
Provision for Loan, Lease, and Other Losses	1,501	2,823	4,318	1,855
Ending Balance	16,829	20,434	16,829	20,434
Ending balance: individually evaluated for impaired	1,808	3,916	1,808	3,916
Ending balance: collectively evaluated for impaired	15,021	16,518	15,021	16,518
Total	1,272,366	1,217,249	1,272,366	1,217,249	1,273,661
Ending balance: individually evaluated for impaired	25,197	37,369	25,197	37,369
Ending balance: collectively evaluated for impaired	1,247,169	1,179,880	1,247,169	1,179,880
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,142	4,209	4,132	4,244
Charge-offs	(914)	(756)	(2,669)	(2,494)
Recoveries	128	132	465	396
Provision for Loan, Lease, and Other Losses	838	635	2,266	2,074
Ending Balance	4,194	4,220	4,194	4,220
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	4,194	4,220	4,194	4,220
Total	604,307	586,278	604,307	586,278	582,218
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	604,307	586,278	604,307	586,278
Unallocated [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,922	5,907	5,926	4,836
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for Loan, Lease, and Other Losses	812	(93)	808	978
Ending Balance	6,734	5,814	6,734	5,814
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	$ 6,734	$ 5,814	$ 6,734	$ 5,814