Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 67,187	$ 61,234
Charge-offs	(10,476)	(4,992)
Recoveries	1,054	703
Provision	4,497	3,787
Ending Balance	62,262	60,732
Ending balance: individually evaluated for impaired	12,689	11,920
Ending balance: collectively evaluated for impaired	49,573	48,812
Total	4,218,810	4,128,588	4,204,704
Ending balance: individually evaluated for impaired	71,421	77,490
Ending balance: collectively evaluated for impaired	4,147,389	4,051,098
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	19,852	18,200
Charge-offs	(538)	(1,914)
Recoveries	128	238
Provision	833	1,619
Ending Balance	20,275	18,143
Ending balance: individually evaluated for impaired	9,846	7,555
Ending balance: collectively evaluated for impaired	10,429	10,588
Total	1,057,663	1,051,209
Ending balance: individually evaluated for impaired	33,322	31,599
Ending balance: collectively evaluated for impaired	1,024,341	1,019,610
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	8,928	6,756
Charge-offs	(84)	(190)
Recoveries	12	56
Provision	(1,123)	(195)
Ending Balance	7,733	6,427
Ending balance: individually evaluated for impaired	503	2,891
Ending balance: collectively evaluated for impaired	7,230	3,536
Total	70,461	64,158
Ending balance: individually evaluated for impaired	8,917	13,800
Ending balance: collectively evaluated for impaired	61,544	50,358
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,908	8,237
Charge-offs	(322)	(1,712)
Recoveries	723	133
Provision	(560)	44
Ending Balance	5,749	6,702
Ending balance: individually evaluated for impaired	1,091	553
Ending balance: collectively evaluated for impaired	4,658	6,149
Total	1,255,515	1,174,572
Ending balance: individually evaluated for impaired	7,813	3,217
Ending balance: collectively evaluated for impaired	1,247,702	1,171,355
Commercial real estate portfolio segment [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	22,441	18,961
Charge-offs	(8,544)	(235)
Recoveries	97	158
Provision	4,476	487
Ending Balance	18,470	19,371
Ending balance: individually evaluated for impaired	1,249	921
Ending balance: collectively evaluated for impaired	17,221	18,450
Total	1,243,676	1,264,060
Ending balance: individually evaluated for impaired	21,369	28,874
Ending balance: collectively evaluated for impaired	1,222,307	1,235,186
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,132	4,244
Charge-offs	(988)	(941)
Recoveries	94	118
Provision	901	831
Ending Balance	4,139	4,252
Ending balance: individually evaluated for impaired	0	0
Ending balance: collectively evaluated for impaired	4,139	4,252
Total	591,495	574,589
Ending balance: individually evaluated for impaired	0	0
Ending balance: collectively evaluated for impaired	591,495	574,589
Unallocated [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,926	4,836
Charge-offs	0	0
Recoveries	0	0
Provision	(30)	1,001
Ending Balance	5,896	5,837
Ending balance: individually evaluated for impaired	0	0
Ending balance: collectively evaluated for impaired	$ 5,896	$ 5,837