Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 46,697	$ 54,506	$ 52,051	$ 54,225
Charge-offs	(4,952)	(8,103)	(12,061)	(11,749)
Recoveries	561	1,005	1,157	1,701
Provision for Loan, Lease, and Other Losses	3,038	3,317	4,197	6,548
Ending Balance	46,697	54,506	52,051	54,225	$ 45,344	$ 52,051	$ 50,725
Ending balance: individually evaluated for impaired					1,675		6,072
Ending balance: collectively evaluated for impaired					43,669		44,653
Total					4,490,854	4,457,308	4,334,214
Ending balance: individually evaluated for impaired					36,744		39,648
Ending balance: collectively evaluated for impaired					4,454,110		4,294,566
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	24,406	26,125	29,627	22,663
Charge-offs	(2,860)	(6,258)	(7,940)	(7,859)
Recoveries	158	336	358	421
Provision for Loan, Lease, and Other Losses	2,051	1,753	1,710	6,731
Ending Balance	24,406	26,125	29,627	22,663	23,755	29,627	21,956
Ending balance: individually evaluated for impaired					1,596		4,995
Ending balance: collectively evaluated for impaired					22,159		16,961
Total					1,098,019	1,052,109	1,062,001
Ending balance: individually evaluated for impaired					21,419		24,573
Ending balance: collectively evaluated for impaired					1,076,600		1,037,428
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	1,528	6,214	2,063	6,600
Charge-offs	0	(296)	0	(296)
Recoveries	84	168	84	337
Provision for Loan, Lease, and Other Losses	(94)	(187)	(629)	(742)
Ending Balance	1,528	6,214	2,063	6,600	1,518	2,063	5,899
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					1,518		5,899
Total					125,010	120,785	100,709
Ending balance: individually evaluated for impaired					0		486
Ending balance: collectively evaluated for impaired					125,010		100,223
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,387	6,026	3,664	7,727
Charge-offs	(484)	(640)	(1,050)	(1,735)
Recoveries	143	79	239	323
Provision for Loan, Lease, and Other Losses	(123)	660	70	(190)
Ending Balance	3,387	6,026	3,664	7,727	2,923	3,664	6,125
Ending balance: individually evaluated for impaired					35		546
Ending balance: collectively evaluated for impaired					2,888		5,579
Total					1,204,499	1,226,344	1,238,791
Ending balance: individually evaluated for impaired					7,700		7,624
Ending balance: collectively evaluated for impaired					1,196,799		1,231,167
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	12,487	11,119	11,881	11,778
Charge-offs	(486)	(157)	(688)	(297)
Recoveries	15	235	153	255
Provision for Loan, Lease, and Other Losses	211	464	881	(75)
Ending Balance	12,487	11,119	11,881	11,778	12,227	11,881	11,661
Ending balance: individually evaluated for impaired					44		531
Ending balance: collectively evaluated for impaired					12,183		11,130
Total					1,416,841	1,405,256	1,306,752
Ending balance: individually evaluated for impaired					7,625		6,965
Ending balance: collectively evaluated for impaired					1,409,216		1,299,787
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,889	5,022	4,816	5,457
Charge-offs	(1,122)	(752)	(2,383)	(1,562)
Recoveries	161	187	323	365
Provision for Loan, Lease, and Other Losses	993	627	2,165	824
Ending Balance	$ 4,889	$ 5,022	$ 4,816	$ 5,457	4,921	4,816	5,084
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					4,921		5,084
Total					646,485	$ 652,814	625,961
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					$ 646,485		$ 625,961