Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 54,506	$ 62,262	$ 54,225	$ 67,187
Charge-offs	(8,103)	(16,136)	(11,749)	(26,612)
Recoveries	1,005	526	1,701	1,580
Provision for Loan, Lease, and Other Losses	3,317	10,800	6,548	15,297
Ending Balance	50,725	57,452	50,725	57,452
Ending balance: individually evaluated for impaired	6,072	9,655	6,072	9,655
Ending balance: collectively evaluated for impaired	44,653	47,797	44,653	47,797
Loans and Leases Receivable, Gross	4,334,214		4,334,214		4,283,833
Total	4,334,214	4,229,752	4,334,214	4,229,752	4,283,833
Ending balance: individually evaluated for impaired	39,648	66,826	39,648	66,826
Ending balance: collectively evaluated for impaired	4,294,566	4,162,926	4,294,566	4,162,926
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	26,125	20,275	22,663	19,852
Charge-offs	(6,258)	(13,683)	(7,859)	(14,221)
Recoveries	336	136	421	264
Provision for Loan, Lease, and Other Losses	1,753	9,847	6,731	10,680
Ending Balance	21,956	16,575	21,956	16,575
Ending balance: individually evaluated for impaired	4,995	6,627	4,995	6,627
Ending balance: collectively evaluated for impaired	16,961	9,948	16,961	9,948
Loans and Leases Receivable, Gross	1,062,001		1,062,001		1,021,056
Total		1,012,315		1,012,315
Ending balance: individually evaluated for impaired	24,573	24,592	24,573	24,592
Ending balance: collectively evaluated for impaired	1,037,428	987,723	1,037,428	987,723
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6,214	7,733	6,600	8,928
Charge-offs	(296)	(671)	(296)	(755)
Recoveries	168	47	337	59
Provision for Loan, Lease, and Other Losses	(187)	1,137	(742)	14
Ending Balance	5,899	8,246	5,899	8,246
Ending balance: individually evaluated for impaired	0	267	0	267
Ending balance: collectively evaluated for impaired	5,899	7,979	5,899	7,979
Loans and Leases Receivable, Gross	100,709		100,709		93,289
Total		66,243		66,243
Ending balance: individually evaluated for impaired	486	3,904	486	3,904
Ending balance: collectively evaluated for impaired	100,223	62,339	100,223	62,339
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6,026	5,749	7,727	5,908
Charge-offs	(640)	(321)	(1,735)	(643)
Recoveries	79	89	323	812
Provision for Loan, Lease, and Other Losses	660	922	(190)	362
Ending Balance	6,125	6,439	6,125	6,439
Ending balance: individually evaluated for impaired	546	1,943	546	1,943
Ending balance: collectively evaluated for impaired	5,579	4,496	5,579	4,496
Loans and Leases Receivable, Gross	1,238,791		1,238,791		1,262,718
Total		1,269,830		1,269,830
Ending balance: individually evaluated for impaired	7,624	10,468	7,624	10,468
Ending balance: collectively evaluated for impaired	1,231,167	1,259,362	1,231,167	1,259,362
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	11,119	18,470	11,778	22,441
Charge-offs	(157)	(694)	(297)	(9,238)
Recoveries	235	11	255	108
Provision for Loan, Lease, and Other Losses	464	(1,659)	(75)	2,817
Ending Balance	11,661	16,128	11,661	16,128
Ending balance: individually evaluated for impaired	531	818	531	818
Ending balance: collectively evaluated for impaired	11,130	15,310	11,130	15,310
Loans and Leases Receivable, Gross	1,306,752		1,306,752		1,296,472
Total		1,280,784		1,280,784
Ending balance: individually evaluated for impaired	6,965	27,862	6,965	27,862
Ending balance: collectively evaluated for impaired	1,299,787	1,252,922	1,299,787	1,252,922
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,022	4,139	5,457	4,132
Charge-offs	(752)	(767)	(1,562)	(1,755)
Recoveries	187	243	365	337
Provision for Loan, Lease, and Other Losses	627	527	824	1,428
Ending Balance	5,084	4,142	5,084	4,142
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	5,084	4,142	5,084	4,142
Loans and Leases Receivable, Gross	625,961		625,961		610,298
Total		600,580		600,580
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	625,961	600,580	625,961	600,580
Unallocated [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	5,896	0	5,926
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for Loan, Lease, and Other Losses	0	26	0	(4)
Ending Balance	0	5,922	0	5,922
Ending balance: individually evaluated for impaired	0	0	0	0
Ending balance: collectively evaluated for impaired	$ 0	$ 5,922	$ 0	$ 5,922