Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 55,222	$ 46,697	$ 50,812	$ 52,051
Charge-offs	(6,138)	(4,952)	(9,646)	(12,061)
Recoveries	365	561	1,757	1,157
Provision for Loan, Lease, and Other Losses	10,372	3,038	16,898	4,197
Ending Balance	55,222	46,697	50,812	52,051	$ 59,821	$ 50,812	$ 45,344
Ending balance: individually evaluated for impaired					15,530		1,675
Ending balance: collectively evaluated for impaired					44,291		43,669
Total					4,843,776	4,683,750	4,490,854
Ending balance: individually evaluated for impaired					55,800		36,744
Ending balance: collectively evaluated for impaired					4,787,976		4,454,110
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	41,721	24,406	31,035	29,627
Charge-offs	(4,753)	(2,860)	(6,145)	(7,940)
Recoveries	64	158	198	358
Provision for Loan, Lease, and Other Losses	9,325	2,051	21,269	1,710
Ending Balance	41,721	24,406	31,035	29,627	46,357	31,035	23,755
Ending balance: individually evaluated for impaired					15,018		1,596
Ending balance: collectively evaluated for impaired					31,339		22,159
Total					1,185,062	1,150,906	1,098,019
Ending balance: individually evaluated for impaired					43,817		21,419
Ending balance: collectively evaluated for impaired					1,141,245		1,076,600
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	901	1,528	887	2,063
Charge-offs	0	0	0	0
Recoveries	4	84	227	84
Provision for Loan, Lease, and Other Losses	(425)	(94)	(634)	(629)
Ending Balance	901	1,528	887	2,063	480	887	1,518
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					480		1,518
Total					242,132	220,736	125,010
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					242,132		125,010
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,628	3,387	2,606	3,664
Charge-offs	(220)	(484)	(602)	(1,050)
Recoveries	142	143	260	239
Provision for Loan, Lease, and Other Losses	55	(123)	341	70
Ending Balance	2,628	3,387	2,606	3,664	2,605	2,606	2,923
Ending balance: individually evaluated for impaired					48		35
Ending balance: collectively evaluated for impaired					2,557		2,888
Total					1,199,005	1,224,465	1,204,499
Ending balance: individually evaluated for impaired					5,966		7,700
Ending balance: collectively evaluated for impaired					1,193,039		1,196,799
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	5,483	12,487	11,924	11,881
Charge-offs	(143)	(486)	(408)	(688)
Recoveries	27	15	783	153
Provision for Loan, Lease, and Other Losses	495	211	(6,437)	881
Ending Balance	5,483	12,487	11,924	11,881	5,862	11,924	12,227
Ending balance: individually evaluated for impaired					464		44
Ending balance: collectively evaluated for impaired					5,398		12,183
Total					1,648,222	1,479,000	1,416,841
Ending balance: individually evaluated for impaired					6,017		7,625
Ending balance: collectively evaluated for impaired					1,642,205		1,409,216
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,489	4,889	4,360	4,816
Charge-offs	(1,022)	(1,122)	(2,491)	(2,383)
Recoveries	128	161	289	323
Provision for Loan, Lease, and Other Losses	922	993	2,359	2,165
Ending Balance	$ 4,489	$ 4,889	$ 4,360	$ 4,816	4,517	4,360	4,921
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					4,517		4,921
Total					569,355	$ 608,643	646,485
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					$ 569,355		$ 646,485