Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses	$ 2,835	$ 1,168	$ 6,930	$ 8,071
Ending Balance					$ 51,061		$ 51,314
Ending balance: individually evaluated for impaired					2,530		7,601
Ending balance: collectively evaluated for impaired					48,531		43,713
Total					6,003,059	$ 5,774,139	5,640,106
Ending balance: individually evaluated for impaired					25,487		35,299
Ending balance: collectively evaluated for impaired					5,977,572		5,604,807
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					4,158		3,683
Total					1,579,441		1,518,850
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					19,312		17,737
Ending balance: individually evaluated for impaired					1,113		662
Ending balance: collectively evaluated for impaired					18,199		17,075
Total					2,118,582		2,172,615
Ending balance: individually evaluated for impaired					10,216		9,037
Ending balance: collectively evaluated for impaired					2,108,366		2,163,578
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					20,693		25,105
Ending balance: individually evaluated for impaired					1,330		6,597
Ending balance: collectively evaluated for impaired					19,363		18,508
Total					1,236,424		1,130,638
Ending balance: individually evaluated for impaired					11,206		21,087
Ending balance: collectively evaluated for impaired					1,225,218		1,109,551
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					2,491		1,262
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					2,491		1,262
Total					441,854		259,825
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					441,854		259,825
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance: individually evaluated for impaired					87		342
Ending balance: collectively evaluated for impaired					4,071		3,341
Ending balance: individually evaluated for impaired					4,065		5,175
Ending balance: collectively evaluated for impaired					1,575,376		1,513,675
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					4,407		3,527
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					4,407		3,527
Total					626,758		558,178
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					626,758		558,178
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	53	35	178	46
Charge-offs	(28)	(132)	(604)	(152)
Recoveries	37	49	81	86
Provision for credit losses	3	204	410	176
Ending Balance	53	35	178	46	65	178	156
Total					452,660	498,843
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	35	13	35	6
Charge-offs	(1)	(32)	(46)	(48)
Recoveries	11	33	35	50
Provision for credit losses	(20)	113	1	119
Ending Balance	35	13	35	6	25	35	127
Total					228,676	248,760
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	2	4	29
Charge-offs	0	0	0	0
Recoveries	0	0	0	0
Provision for credit losses	25	4	21	(23)
Ending Balance	0	2	4	29	25	4	6
Total					178,974	201,195
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	18	20	139	11
Charge-offs	(26)	(93)	(552)	(93)
Recoveries	21	11	32	18
Provision for credit losses	2	85	396	87
Ending Balance	18	20	139	11	15	139	23
Total					34,719	37,526
Acquired Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	6
Provision for credit losses	0	0	0	(6)
Ending Balance	0	0	0	0	0	0	0
Total					6,469	5,970
Acquired Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	(1)	(7)	(6)	(11)
Recoveries	5	5	14	12
Provision for credit losses	(4)	2	(8)	(1)
Ending Balance	0	0	0	0	0	0	0
Total					3,822	5,392
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	49,600	53,697	47,586	48,252
Charge-offs	(1,825)	(4,116)	(3,853)	(6,198)
Recoveries	389	613	743	1,209
Provision for credit losses	2,832	964	6,520	7,895
Ending Balance	49,600	53,697	47,586	48,252	50,996	47,586	51,158
Total					5,550,399	5,275,296
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,150	3,141	3,934	2,753
Charge-offs	(98)	(226)	(234)	(681)
Recoveries	103	121	184	196
Provision for credit losses	(22)	520	249	1,288
Ending Balance	4,150	3,141	3,934	2,753	4,133	3,934	3,556
Total					1,350,765	1,313,645
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,218	18,494	18,382	17,328
Charge-offs	0	(2,243)	(299)	(2,411)
Recoveries	38	18	79	87
Provision for credit losses	31	1,462	1,125	2,727
Ending Balance	19,218	18,494	18,382	17,328	19,287	18,382	17,731
Total					1,939,608	1,922,349
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,815	27,532	19,235	23,418
Charge-offs	(359)	(571)	(842)	(861)
Recoveries	63	362	139	618
Provision for credit losses	1,159	(2,241)	2,146	1,907
Ending Balance	19,815	27,532	19,235	23,418	20,678	19,235	25,082
Total					1,201,705	1,100,947
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,254	1,114	2,002	1,349
Charge-offs	0	0	0	0
Recoveries	42	0	84	1
Provision for credit losses	195	148	405	(88)
Ending Balance	2,254	1,114	2,002	1,349	2,491	2,002	1,262
Total					435,385	353,008
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,163	3,416	4,033	3,404
Charge-offs	(1,368)	(1,076)	(2,478)	(2,245)
Recoveries	143	112	257	307
Provision for credit losses	1,469	1,075	2,595	2,061
Ending Balance	$ 4,163	$ 3,416	$ 4,033	$ 3,404	4,407	4,033	$ 3,527
Total					$ 622,936	$ 585,347