Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Provision for credit losses	$ 1,168	$ (1,609)	$ 8,071	$ 1,620
Ending Balance					$ 51,314		$ 48,067
Ending balance: individually evaluated for impaired					7,601		2,135
Ending balance: collectively evaluated for impaired					43,713		45,932
Total					5,640,106	$ 5,407,376	5,374,782
Ending balance: individually evaluated for impaired					35,299		32,592
Ending balance: collectively evaluated for impaired					5,604,807		5,342,190
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					3,683		2,120
Total					1,518,850		1,416,926
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					17,737		7,357
Ending balance: individually evaluated for impaired					662		309
Ending balance: collectively evaluated for impaired					17,075		7,048
Total					2,172,615		1,963,001
Ending balance: individually evaluated for impaired					9,037		8,697
Ending balance: collectively evaluated for impaired					2,163,578		1,954,304
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					25,105		33,490
Ending balance: individually evaluated for impaired					6,597		1,791
Ending balance: collectively evaluated for impaired					18,508		31,699
Total					1,130,638		1,199,800
Ending balance: individually evaluated for impaired					21,087		16,917
Ending balance: collectively evaluated for impaired					1,109,551		1,182,883
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					1,262		768
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					1,262		768
Total					259,825		249,255
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					259,825		249,255
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending balance: individually evaluated for impaired					342		35
Ending balance: collectively evaluated for impaired					3,341		2,085
Ending balance: individually evaluated for impaired					5,175		6,978
Ending balance: collectively evaluated for impaired					1,513,675		1,409,948
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					3,527		4,332
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					3,527		4,332
Total					558,178		545,800
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					558,178		545,800
Acquired Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	35	31	46	19
Charge-offs	(132)	(2)	(152)	(17)
Recoveries	49	71	86	78
Provision for credit losses	204	72	176	92
Ending Balance	35	31	46	19	156	46	172
Total					550,049	400,015
Acquired Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	13	31	6	19
Charge-offs	(32)	(1)	(48)	(9)
Recoveries	33	21	50	27
Provision for credit losses	113	(47)	119	(33)
Ending Balance	13	31	6	19	127	6	4
Total					267,497	220,251
Acquired Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	2	0	29	0
Charge-offs	0	0	0	0
Recoveries	0	4	0	4
Provision for credit losses	4	46	(23)	46
Ending Balance	2	0	29	0	6	29	50
Total					224,328	126,911
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	20	0	11	0
Charge-offs	(93)	0	(93)	0
Recoveries	11	0	18	0
Provision for credit losses	85	118	87	118
Ending Balance	20	0	11	0	23	11	118
Total					42,563	40,642
Acquired Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	1	6	1
Provision for credit losses	0	(1)	(6)	(1)
Ending Balance	0	0	0	0	0	0	0
Total					9,130	5,963
Acquired Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	(7)	(1)	(11)	(8)
Recoveries	5	45	12	46
Provision for credit losses	2	(44)	(1)	(38)
Ending Balance	0	0	0	0	0	0	0
Total					6,531	6,248
Originated Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	53,697	48,645	48,252	50,166
Charge-offs	(4,116)	(2,598)	(6,198)	(8,117)
Recoveries	613	3,529	1,209	4,318
Provision for credit losses	964	(1,681)	7,895	1,528
Ending Balance	53,697	48,645	48,252	50,166	51,158	48,252	47,895
Total					5,090,057	5,007,361
Originated Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,141	2,414	2,753	2,492
Charge-offs	(226)	(145)	(681)	(610)
Recoveries	121	70	196	192
Provision for credit losses	520	(223)	1,288	42
Ending Balance	3,141	2,414	2,753	2,492	3,556	2,753	2,116
Total					1,251,353	1,206,119
Originated Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	18,494	6,582	17,328	6,619
Charge-offs	(2,243)	(29)	(2,411)	(60)
Recoveries	18	29	87	146
Provision for credit losses	1,462	725	2,727	602
Ending Balance	18,494	6,582	17,328	6,619	17,731	17,328	7,307
Total					1,948,287	1,892,185
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	27,532	34,701	23,418	35,974
Charge-offs	(571)	(1,452)	(861)	(5,277)
Recoveries	362	3,268	618	3,636
Provision for credit losses	(2,241)	(3,145)	1,907	(961)
Ending Balance	27,532	34,701	23,418	35,974	25,082	23,418	33,372
Total					1,088,075	1,122,741
Originated Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	1,114	614	1,349	577
Charge-offs	0	0	0	0
Recoveries	0	42	1	96
Provision for credit losses	148	112	(88)	95
Ending Balance	1,114	614	1,349	577	1,262	1,349	768
Total					250,695	242,905
Originated Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,416	4,334	3,404	4,504
Charge-offs	(1,076)	(972)	(2,245)	(2,170)
Recoveries	112	120	307	248
Provision for credit losses	1,075	850	2,061	1,750
Ending Balance	$ 3,416	$ 4,334	$ 3,404	$ 4,504	3,527	3,404	$ 4,332
Total					$ 551,647	$ 543,411