Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Dec. 31, 2016
Allowance for Doubtful Accounts Receivable [Roll Forward]
Provision for credit losses	$ (1,609)	$ 10,372	$ 1,620	$ 16,898
Ending Balance		59,821		59,821	$ 48,067
Ending balance: individually evaluated for impaired		15,530		15,530	2,135
Ending balance: collectively evaluated for impaired		44,291		44,291	45,932
Total		4,843,776		4,843,776	5,374,782	$ 4,879,347
Ending balance: individually evaluated for impaired		55,800		55,800	32,592
Ending balance: collectively evaluated for impaired		4,787,976		4,787,976	5,342,190
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		2,605		2,605	2,120
Total		1,199,005		1,199,005	1,416,926
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		5,862		5,862	7,357
Ending balance: individually evaluated for impaired		464		464	309
Ending balance: collectively evaluated for impaired		5,398		5,398	7,048
Total		1,648,222		1,648,222	1,963,001
Ending balance: individually evaluated for impaired		6,017		6,017	8,697
Ending balance: collectively evaluated for impaired		1,642,205		1,642,205	1,954,304
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		46,357		46,357	33,490
Ending balance: individually evaluated for impaired		15,018		15,018	1,791
Ending balance: collectively evaluated for impaired		31,339		31,339	31,699
Total		1,185,062		1,185,062	1,199,800
Ending balance: individually evaluated for impaired		43,817		43,817	16,917
Ending balance: collectively evaluated for impaired		1,141,245		1,141,245	1,182,883
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		480		480	768
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		480		480	768
Total		242,132		242,132	249,255
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		242,132		242,132	249,255
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending balance: individually evaluated for impaired		48		48	35
Ending balance: collectively evaluated for impaired		2,557		2,557	2,085
Ending balance: individually evaluated for impaired		5,966		5,966	6,978
Ending balance: collectively evaluated for impaired		1,193,039		1,193,039	1,409,948
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance		4,517		4,517	4,332
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		4,517		4,517	4,332
Total		569,355		569,355	545,800
Ending balance: individually evaluated for impaired		0		0	0
Ending balance: collectively evaluated for impaired		569,355		569,355	545,800
Acquired Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	31		19
Charge-offs	(2)		(17)
Recoveries	71		78
Provision for credit losses	72		92
Ending Balance	31		19		172	19
Total					455,579	100,876
Acquired Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	31		19
Charge-offs	(1)		(9)
Recoveries	21		27
Provision for credit losses	(47)		(33)
Ending Balance	31		19		4	19
Total					247,709	63,341
Acquired Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0		0
Charge-offs	0		0
Recoveries	4		4
Provision for credit losses	46		46
Ending Balance	0		0		50	0
Total					143,411	25,167
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0		0
Charge-offs	0		0
Recoveries	0		0
Provision for credit losses	118		118
Ending Balance	0		0		118	0
Total					42,258	8,399
Acquired Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0		0
Charge-offs	0		0
Recoveries	1		1
Provision for credit losses	(1)		(1)
Ending Balance	0		0		0	0
Total					12,220	1,781
Acquired Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0		0
Charge-offs	(1)		(8)
Recoveries	45		46
Provision for credit losses	(44)		(38)
Ending Balance	0		0		0	0
Total					9,981	2,188
Originated Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	48,645	55,222	50,166	50,812
Charge-offs	(2,598)	(6,138)	(8,117)	(9,646)
Recoveries	3,529	365	4,318	1,757
Provision for credit losses	(1,681)	10,372	1,528	16,898
Ending Balance	48,645	55,222	50,166	50,812	47,895	50,166
Total					4,919,203	4,778,471
Originated Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2,414	2,628	2,492	2,606
Charge-offs	(145)	(220)	(610)	(602)
Recoveries	70	142	192	260
Provision for credit losses	(223)	55	42	341
Ending Balance	2,414	2,628	2,492	2,606	2,116	2,492
Total					1,169,217	1,165,851
Originated Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6,582	5,483	6,619	11,924
Charge-offs	(29)	(143)	(60)	(408)
Recoveries	29	27	146	783
Provision for credit losses	725	495	602	(6,437)
Ending Balance	6,582	5,483	6,619	11,924	7,307	6,619
Total					1,819,590	1,717,043
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	34,701	41,721	35,974	31,035
Charge-offs	(1,452)	(4,753)	(5,277)	(6,145)
Recoveries	3,268	64	3,636	198
Provision for credit losses	(3,145)	9,325	(961)	21,269
Ending Balance	34,701	41,721	35,974	31,035	33,372	35,974
Total					1,157,542	1,131,148
Originated Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	614	901	577	887
Charge-offs	0	0	0	0
Recoveries	42	4	96	227
Provision for credit losses	112	(425)	95	(634)
Ending Balance	614	901	577	887	768	577
Total					237,035	217,840
Originated Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	4,334	4,489	4,504	4,360
Charge-offs	(972)	(1,022)	(2,170)	(2,491)
Recoveries	120	128	248	289
Provision for credit losses	850	922	1,750	2,359
Ending Balance	$ 4,334	$ 4,489	$ 4,504	$ 4,360	4,332	4,504
Total					$ 535,819	$ 546,589