Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 50,812	$ 52,051
Charge-offs	(3,508)	(7,109)
Recoveries	1,392	596
Provision for Loan, Lease, and Other Losses	6,526	1,159
Ending Balance	50,812	52,051	$ 55,222	$ 50,812	$ 46,697
Ending balance: individually evaluated for impaired			13,395		3,829
Ending balance: collectively evaluated for impaired			41,827		42,868
Total			4,798,755	4,683,750	4,437,601
Ending balance: individually evaluated for impaired			54,196		39,659
Ending balance: collectively evaluated for impaired			4,744,559		4,397,942
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	31,035	29,627
Charge-offs	(1,392)	(5,080)
Recoveries	134	200
Provision for Loan, Lease, and Other Losses	11,944	(341)
Ending Balance	31,035	29,627	41,721	31,035	24,406
Ending balance: individually evaluated for impaired			12,900		3,397
Ending balance: collectively evaluated for impaired			28,821		21,009
Total			1,190,384	1,150,906	1,066,788
Ending balance: individually evaluated for impaired			42,016		24,586
Ending balance: collectively evaluated for impaired			1,148,368		1,042,202
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	887	2,063
Charge-offs	0	0
Recoveries	223	0
Provision for Loan, Lease, and Other Losses	(209)	(535)
Ending Balance	887	2,063	901	887	1,528
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			901		1,528
Total			256,856	220,736	107,882
Ending balance: individually evaluated for impaired			0		199
Ending balance: collectively evaluated for impaired			256,856		107,683
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,606	3,664
Charge-offs	(382)	(566)
Recoveries	118	96
Provision for Loan, Lease, and Other Losses	286	193
Ending Balance	2,606	3,664	2,628	2,606	3,387
Ending balance: individually evaluated for impaired			75		165
Ending balance: collectively evaluated for impaired			2,553		3,222
Total			1,212,962	1,224,465	1,210,511
Ending balance: individually evaluated for impaired			6,246		7,071
Ending balance: collectively evaluated for impaired			1,206,716		1,203,440
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	11,924	11,881
Charge-offs	(265)	(202)
Recoveries	756	138
Provision for Loan, Lease, and Other Losses	(6,932)	670
Ending Balance	11,924	11,881	5,483	11,924	12,487
Ending balance: individually evaluated for impaired			420		267
Ending balance: collectively evaluated for impaired			5,063		12,220
Total			1,552,904	1,479,000	1,400,276
Ending balance: individually evaluated for impaired			5,934		7,803
Ending balance: collectively evaluated for impaired			1,546,970		1,392,473
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,360	4,816
Charge-offs	(1,469)	(1,261)
Recoveries	161	162
Provision for Loan, Lease, and Other Losses	1,437	1,172
Ending Balance	$ 4,360	$ 4,816	4,489	4,360	4,889
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			4,489		4,889
Total			585,649	$ 608,643	652,144
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			$ 585,649		$ 652,144