Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 59,821	$ 45,344	$ 50,812	$ 52,051
Charge-offs	(8,807)	(2,401)	(18,453)	(14,462)
Recoveries	312	954	2,069	2,111
Provision for Loan, Lease, and Other Losses	3,408	4,621	20,306	8,818
Ending Balance	59,821	45,344	50,812	52,051	$ 54,734	$ 50,812	$ 48,518
Ending balance: individually evaluated for impaired					8,175		4,255
Ending balance: collectively evaluated for impaired					46,559		44,263
Total					4,860,652	4,683,750	4,575,735
Ending balance: individually evaluated for impaired					46,252		34,595
Ending balance: collectively evaluated for impaired					4,814,400		4,541,140
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	46,357	23,755	31,035	29,627
Charge-offs	(7,163)	(639)	(13,308)	(8,579)
Recoveries	63	564	261	922
Provision for Loan, Lease, and Other Losses	2,666	3,520	23,935	5,230
Ending Balance	46,357	23,755	31,035	29,627	41,923	31,035	27,200
Ending balance: individually evaluated for impaired					7,739		4,202
Ending balance: collectively evaluated for impaired					34,184		22,998
Total					1,207,447	1,150,906	1,126,881
Ending balance: individually evaluated for impaired					35,501		22,852
Ending balance: collectively evaluated for impaired					1,171,946		1,104,029
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	480	1,518	887	2,063
Charge-offs	0	0	0	0
Recoveries	0	0	227	84
Provision for Loan, Lease, and Other Losses	(4)	75	(638)	(554)
Ending Balance	480	1,518	887	2,063	476	887	1,593
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					476		1,593
Total					229,375	220,736	179,710
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					229,375		179,710
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,605	2,923	2,606	3,664
Charge-offs	(374)	(301)	(976)	(1,351)
Recoveries	147	178	407	417
Provision for Loan, Lease, and Other Losses	(11)	(124)	330	(54)
Ending Balance	2,605	2,923	2,606	3,664	2,367	2,606	2,676
Ending balance: individually evaluated for impaired					6		20
Ending balance: collectively evaluated for impaired					2,361		2,656
Total					1,185,759	1,224,465	1,204,220
Ending balance: individually evaluated for impaired					5,670		6,037
Ending balance: collectively evaluated for impaired					1,180,089		1,198,183
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,862	12,227	11,924	11,881
Charge-offs	(10)	(561)	(418)	(1,249)
Recoveries	20	33	803	186
Provision for Loan, Lease, and Other Losses	(288)	703	(6,725)	1,584
Ending Balance	5,862	12,227	11,924	11,881	5,584	11,924	12,402
Ending balance: individually evaluated for impaired					430		33
Ending balance: collectively evaluated for impaired					5,154		12,369
Total					1,683,015	1,479,000	1,435,954
Ending balance: individually evaluated for impaired					5,081		5,706
Ending balance: collectively evaluated for impaired					1,677,934		1,430,248
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,517	4,921	4,360	4,816
Charge-offs	(1,260)	(900)	(3,751)	(3,283)
Recoveries	82	179	371	502
Provision for Loan, Lease, and Other Losses	1,045	447	3,404	2,612
Ending Balance	$ 4,517	$ 4,921	$ 4,360	$ 4,816	4,384	4,360	4,647
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					4,384		4,647
Total					555,056	$ 608,643	628,970
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					$ 555,056		$ 628,970