Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Provision for credit losses	$ 2,708	$ 2,961	$ 9,638	$ 11,032
Ending Balance					$ 50,035		$ 50,746
Ending balance: individually evaluated for impaired					1,546		5,363
Ending balance: collectively evaluated for impaired					48,489		45,383
Total					6,099,561	$ 5,774,139	5,662,782
Ending balance: individually evaluated for impaired					25,344		29,398
Ending balance: collectively evaluated for impaired					6,074,217		5,633,384
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					3,917		3,657
Total					1,666,220		1,533,338
Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					19,048		19,671
Ending balance: individually evaluated for impaired					488		1,722
Ending balance: collectively evaluated for impaired					18,560		17,949
Total					2,124,240		2,136,431
Ending balance: individually evaluated for impaired					10,825		12,012
Ending balance: collectively evaluated for impaired					2,113,415		2,124,419
Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					19,947		22,009
Ending balance: individually evaluated for impaired					1,054		3,474
Ending balance: collectively evaluated for impaired					18,893		18,535
Total					1,210,936		1,116,204
Ending balance: individually evaluated for impaired					10,417		12,864
Ending balance: collectively evaluated for impaired					1,200,519		1,103,340
Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					2,410		1,592
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					2,410		1,592
Total					420,281		298,395
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					420,281		298,395
Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending balance: individually evaluated for impaired					4		167
Ending balance: collectively evaluated for impaired					3,913		3,490
Ending balance: individually evaluated for impaired					4,102		4,522
Ending balance: collectively evaluated for impaired					1,662,118		1,528,816
Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Ending Balance					4,713		3,817
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					4,713		3,817
Total					677,884		578,414
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					677,884		578,414
Acquired Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	65	156	178	46
Charge-offs	(1,428)	(13)	(2,032)	(165)
Recoveries	32	50	113	136
Provision for credit losses	1,360	(118)	1,770	58
Ending Balance	65	156	178	46	29	178	75
Total					522,072	498,843
Acquired Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	25	127	35	6
Charge-offs	0	(9)	(46)	(57)
Recoveries	11	25	46	75
Provision for credit losses	(35)	(96)	(34)	23
Ending Balance	25	127	35	6	1	35	47
Total					285,734	248,760
Acquired Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	25	6	4	29
Charge-offs	(1,376)	0	(1,376)	0
Recoveries	0	0	0	0
Provision for credit losses	1,372	2	1,393	(21)
Ending Balance	25	6	4	29	21	4	8
Total					176,707	201,195
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	15	23	139	11
Charge-offs	(49)	0	(601)	(93)
Recoveries	21	13	53	31
Provision for credit losses	20	(16)	416	71
Ending Balance	15	23	139	11	7	139	20
Total					37,866	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					6,719	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	(3)	(4)	(9)	(15)
Recoveries	0	12	14	24
Provision for credit losses	3	(8)	(5)	(9)
Ending Balance	0	0	0	0	0	0	0
Total					15,046	5,392
Originated Loans [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	50,996	51,158	47,586	48,252
Charge-offs	(2,702)	(3,926)	(6,555)	(10,124)
Recoveries	364	360	1,107	1,569
Provision for credit losses	1,348	3,079	7,868	10,974
Ending Balance	50,996	51,158	47,586	48,252	50,006	47,586	50,671
Total					5,577,489	5,275,296
Originated Loans [Member] | Residential real estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,133	3,556	3,934	2,753
Charge-offs	(383)	(268)	(617)	(949)
Recoveries	6	26	190	222
Provision for credit losses	160	296	409	1,584
Ending Balance	4,133	3,556	3,934	2,753	3,916	3,934	3,610
Total					1,380,486	1,313,645
Originated Loans [Member] | Commercial Real Estate [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,287	17,731	18,382	17,328
Charge-offs	(6)	0	(305)	(2,411)
Recoveries	81	36	160	123
Provision for credit losses	(335)	1,896	790	4,623
Ending Balance	19,287	17,731	18,382	17,328	19,027	18,382	19,663
Total					1,947,533	1,922,349
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	20,678	25,082	19,235	23,418
Charge-offs	(742)	(2,582)	(1,584)	(3,443)
Recoveries	41	53	180	671
Provision for credit losses	(37)	(564)	2,109	1,343
Ending Balance	20,678	25,082	19,235	23,418	19,940	19,235	21,989
Total					1,173,070	1,100,947
Originated Loans [Member] | Real estate construction [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,491	1,262	2,002	1,349
Charge-offs	0	0	0	0
Recoveries	74	92	158	93
Provision for credit losses	(155)	238	250	150
Ending Balance	2,491	1,262	2,002	1,349	2,410	2,002	1,592
Total					413,562	353,008
Originated Loans [Member] | Loans to individuals [Member]
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,407	3,527	4,033	3,404
Charge-offs	(1,571)	(1,076)	(4,049)	(3,321)
Recoveries	162	153	419	460
Provision for credit losses	1,715	1,213	4,310	3,274
Ending Balance	$ 4,407	$ 3,527	$ 4,033	$ 3,404	4,713	4,033	$ 3,817
Total					$ 662,838	$ 585,347