Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	$ 50,812	$ 52,051	$ 54,225
Charge-offs		(18,868)	(17,472)
Recoveries		2,681	4,102
Provision for credit losses	18,480	14,948	11,196
Ending Balance	50,185	50,812	52,051
Ending balance: individually evaluated for impaired	3,144	7,045	9,461
Ending balance: collectively evaluated for impaired	47,041	43,767	42,590
Ending balance	4,879,347	4,683,750	4,457,308
Ending balance: individually evaluated for impaired	33,668	44,009	48,442
Ending balance: collectively evaluated for impaired	4,845,679	4,639,741	4,408,866
Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	31,035	29,627	22,663
Charge-offs		(11,429)	(8,911)
Recoveries		1,097	734
Provision for credit losses		11,740	15,141
Ending Balance	35,974	31,035	29,627
Ending balance: individually evaluated for impaired	2,530	6,952	9,304
Ending balance: collectively evaluated for impaired	33,444	24,083	20,323
Ending balance	1,139,547	1,150,906	1,052,109
Ending balance: individually evaluated for impaired	22,325	30,767	33,332
Ending balance: collectively evaluated for impaired	1,117,222	1,120,139	1,018,777
Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	887	2,063	6,600
Charge-offs		(8)	(296)
Recoveries		84	1,340
Provision for credit losses		(1,252)	(5,581)
Ending Balance	577	887	2,063
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	577	887	2,063
Ending balance	219,621	220,736	120,785
Ending balance: individually evaluated for impaired	0	0	193
Ending balance: collectively evaluated for impaired	219,621	220,736	120,592
Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,606	3,664	7,727
Charge-offs		(1,539)	(3,153)
Recoveries		587	650
Provision for credit losses		(106)	(1,560)
Ending Balance	2,511	2,606	3,664
Ending balance: individually evaluated for impaired	180	51	56
Ending balance: collectively evaluated for impaired	2,331	2,555	3,608
Ending balance	1,229,192	1,224,465	1,226,344
Ending balance: individually evaluated for impaired	5,875	6,099	7,127
Ending balance: collectively evaluated for impaired	1,223,317	1,218,366	1,219,217
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,924	11,881	11,778
Charge-offs		(1,538)	(1,148)
Recoveries		229	612
Provision for credit losses		1,352	639
Ending Balance	6,619	11,924	11,881
Ending balance: individually evaluated for impaired	434	42	101
Ending balance: collectively evaluated for impaired	6,185	11,882	11,780
Ending balance	1,742,210	1,479,000	1,405,256
Ending balance: individually evaluated for impaired	5,468	7,143	7,790
Ending balance: collectively evaluated for impaired	1,736,742	1,471,857	1,397,466
Loans To Individuals [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	4,360	4,816	5,457
Charge-offs		(4,354)	(3,964)
Recoveries		684	766
Provision for credit losses		3,214	2,557
Ending Balance	4,504	4,360	4,816
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	4,504	4,360	4,816
Ending balance	548,777	608,643	652,814
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	548,777	608,643	$ 652,814
Originated Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	50,812
Charge-offs	(26,305)
Recoveries	7,198
Provision for credit losses	18,461
Ending Balance	50,166	50,812
Ending balance	4,778,471
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	31,035
Charge-offs	(19,603)
Recoveries	4,164
Provision for credit losses	20,378
Ending Balance	35,974	31,035
Ending balance	1,131,148
Originated Loans [Member] | Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	887
Charge-offs	0
Recoveries	562
Provision for credit losses	(872)
Ending Balance	577	887
Ending balance	217,840
Originated Loans [Member] | Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	2,606
Charge-offs	(1,189)
Recoveries	481
Provision for credit losses	594
Ending Balance	2,492	2,606
Ending balance	1,165,851
Originated Loans [Member] | Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	11,924
Charge-offs	(570)
Recoveries	1,522
Provision for credit losses	(6,257)
Ending Balance	6,619	11,924
Ending balance	1,717,043
Originated Loans [Member] | Loans To Individuals [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Beginning Balance	4,360
Charge-offs	(4,943)
Recoveries	469
Provision for credit losses	4,618
Ending Balance	4,504	$ 4,360
Ending balance	546,589
Acquired Loans [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending Balance	19
Ending balance	100,876
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending Balance	0
Ending balance	8,399
Acquired Loans [Member] | Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending Balance	0
Ending balance	1,781
Acquired Loans [Member] | Residential real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending Balance	19
Ending balance	63,341
Acquired Loans [Member] | Commercial real estate [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending Balance	0
Ending balance	25,167
Acquired Loans [Member] | Loans To Individuals [Member]
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Ending Balance	0
Ending balance	$ 2,188