Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 45,344	$ 50,725	$ 52,051	$ 54,225
Charge-offs	(2,401)	(2,880)	(14,462)	(14,629)
Recoveries	954	866	2,111	2,567
Provision for Loan, Lease, and Other Losses	4,621	2,073	8,818	8,621
Ending Balance	45,344	50,725	52,051	54,225	$ 48,518	$ 52,051	$ 50,784
Ending balance: individually evaluated for impaired					4,255		4,632
Ending balance: collectively evaluated for impaired					44,263		46,152
Total					4,575,735	4,457,308	4,411,481
Ending balance: individually evaluated for impaired					34,595		37,716
Ending balance: collectively evaluated for impaired					4,541,140		4,373,765
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	23,755	21,956	29,627	22,663
Charge-offs	(639)	(498)	(8,579)	(8,357)
Recoveries	564	204	922	625
Provision for Loan, Lease, and Other Losses	3,520	(1,958)	5,230	4,773
Ending Balance	23,755	21,956	29,627	22,663	27,200	29,627	19,704
Ending balance: individually evaluated for impaired					4,202		4,271
Ending balance: collectively evaluated for impaired					22,998		15,433
Total					1,126,881	1,052,109	1,071,531
Ending balance: individually evaluated for impaired					22,852		23,773
Ending balance: collectively evaluated for impaired					1,104,029		1,047,758
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	1,518	5,899	2,063	6,600
Charge-offs	0	0	0	(296)
Recoveries	0	132	84	469
Provision for Loan, Lease, and Other Losses	75	2,073	(554)	1,331
Ending Balance	1,518	5,899	2,063	6,600	1,593	2,063	8,104
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					1,593		8,104
Total					179,710	120,785	115,788
Ending balance: individually evaluated for impaired					0		199
Ending balance: collectively evaluated for impaired					179,710		115,589
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,923	6,125	3,664	7,727
Charge-offs	(301)	(551)	(1,351)	(2,286)
Recoveries	178	97	417	420
Provision for Loan, Lease, and Other Losses	(124)	(217)	(54)	(407)
Ending Balance	2,923	6,125	3,664	7,727	2,676	3,664	5,454
Ending balance: individually evaluated for impaired					20		332
Ending balance: collectively evaluated for impaired					2,656		5,122
Total					1,204,220	1,226,344	1,234,842
Ending balance: individually evaluated for impaired					6,037		6,854
Ending balance: collectively evaluated for impaired					1,198,183		1,227,988
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	12,227	11,661	11,881	11,778
Charge-offs	(561)	(812)	(1,249)	(1,109)
Recoveries	33	177	186	432
Provision for Loan, Lease, and Other Losses	703	1,528	1,584	1,453
Ending Balance	12,227	11,661	11,881	11,778	12,402	11,881	12,554
Ending balance: individually evaluated for impaired					33		29
Ending balance: collectively evaluated for impaired					12,369		12,525
Total					1,435,954	1,405,256	1,345,302
Ending balance: individually evaluated for impaired					5,706		6,890
Ending balance: collectively evaluated for impaired					1,430,248		1,338,412
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,921	5,084	4,816	5,457
Charge-offs	(900)	(1,019)	(3,283)	(2,581)
Recoveries	179	256	502	621
Provision for Loan, Lease, and Other Losses	447	647	2,612	1,471
Ending Balance	$ 4,921	$ 5,084	$ 4,816	$ 5,457	4,647	4,816	4,968
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					4,647		4,968
Total					628,970	$ 652,814	644,018
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					$ 628,970		$ 644,018