Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance		$ 50,812
Charge-offs		(3,508)
Recoveries		1,392
Provision for credit losses	$ 3,229	6,526
Ending Balance		50,812	$ 48,676		$ 55,222
Ending balance: individually evaluated for impaired			3,017		13,395
Ending balance: collectively evaluated for impaired			45,659		41,827
Total			4,907,961	$ 4,879,347	4,798,755
Ending balance: individually evaluated for impaired			38,366		54,196
Ending balance: collectively evaluated for impaired			4,869,595		4,744,559
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance		2,606
Charge-offs		(382)
Recoveries		118
Provision for credit losses		286
Ending Balance		2,606	2,445		2,628
Ending balance: individually evaluated for impaired					75
Ending balance: collectively evaluated for impaired					2,553
Total			1,217,398		1,212,962
Ending balance: individually evaluated for impaired					6,246
Ending balance: collectively evaluated for impaired					1,206,716
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance		11,924
Charge-offs		(265)
Recoveries		756
Provision for credit losses		(6,932)
Ending Balance		11,924	6,582		5,483
Ending balance: individually evaluated for impaired			376		420
Ending balance: collectively evaluated for impaired			6,206		5,063
Total			1,761,101		1,552,904
Ending balance: individually evaluated for impaired			5,819		5,934
Ending balance: collectively evaluated for impaired			1,755,282		1,546,970
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance		31,035
Charge-offs		(1,392)
Recoveries		134
Provision for credit losses		11,944
Ending Balance		31,035	34,701		41,721
Ending balance: individually evaluated for impaired			2,466		12,900
Ending balance: collectively evaluated for impaired			32,235		28,821
Total			1,148,460		1,190,384
Ending balance: individually evaluated for impaired			26,260		42,016
Ending balance: collectively evaluated for impaired			1,122,200		1,148,368
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance		887
Charge-offs		0
Recoveries		223
Provision for credit losses		(209)
Ending Balance		887	614		901
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			614		901
Total			240,122		256,856
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			240,122		256,856
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending balance: individually evaluated for impaired			175
Ending balance: collectively evaluated for impaired			2,270
Ending balance: individually evaluated for impaired			6,287
Ending balance: collectively evaluated for impaired			1,211,111
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance		4,360
Charge-offs		(1,469)
Recoveries		161
Provision for credit losses		1,437
Ending Balance		$ 4,360	4,334		4,489
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			4,334		4,489
Total			540,880		585,649
Ending balance: individually evaluated for impaired			0		0
Ending balance: collectively evaluated for impaired			540,880		$ 585,649
Acquired Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	19
Charge-offs	(15)
Recoveries	7
Provision for credit losses	20
Ending Balance	19		31	19
Total			94,686	100,876
Acquired Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	19
Charge-offs	(8)
Recoveries	6
Provision for credit losses	14
Ending Balance	19		31	19
Total			60,162	63,341
Acquired Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0
Charge-offs	0
Recoveries	0
Provision for credit losses	0
Ending Balance	0		0	0
Total			24,738	25,167
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0
Charge-offs	0
Recoveries	0
Provision for credit losses	0
Ending Balance	0		0	0
Total			7,758	8,399
Acquired Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0
Charge-offs	0
Recoveries	0
Provision for credit losses	0
Ending Balance	0		0	0
Total			139	1,781
Acquired Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0
Charge-offs	(7)
Recoveries	1
Provision for credit losses	6
Ending Balance	0		0	0
Total			1,889	2,188
Originated Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	50,166
Charge-offs	(5,519)
Recoveries	789
Provision for credit losses	3,209
Ending Balance	50,166		48,645	50,166
Total			4,813,275	4,778,471
Originated Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,492
Charge-offs	(465)
Recoveries	122
Provision for credit losses	265
Ending Balance	2,492		2,414	2,492
Total			1,157,236	1,165,851
Originated Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6,619
Charge-offs	(31)
Recoveries	117
Provision for credit losses	(123)
Ending Balance	6,619		6,582	6,619
Total			1,736,363	1,717,043
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	35,974
Charge-offs	(3,825)
Recoveries	368
Provision for credit losses	2,184
Ending Balance	35,974		34,701	35,974
Total			1,140,702	1,131,148
Originated Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	577
Charge-offs	0
Recoveries	54
Provision for credit losses	(17)
Ending Balance	577		614	577
Total			239,983	217,840
Originated Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,504
Charge-offs	(1,198)
Recoveries	128
Provision for credit losses	900
Ending Balance	$ 4,504		4,334	4,504
Total			$ 538,991	$ 546,589