Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Allowance for Doubtful Accounts Receivable [Roll Forward]
Provision for credit losses	$ 2,961	$ 1,214	$ 11,032	$ 2,834
Ending Balance					$ 50,746		$ 48,176
Ending balance: individually evaluated for impaired					5,363		2,181
Ending balance: collectively evaluated for impaired					45,383		45,995
Total					5,662,782	$ 5,407,376	5,375,847
Ending balance: individually evaluated for impaired					29,398		31,326
Ending balance: collectively evaluated for impaired					5,633,384		5,344,521
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					3,657		2,898
Total					1,533,338		1,423,422
Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					19,671		17,367
Ending balance: individually evaluated for impaired					1,722		261
Ending balance: collectively evaluated for impaired					17,949		17,106
Total					2,136,431		1,990,264
Ending balance: individually evaluated for impaired					12,012		8,189
Ending balance: collectively evaluated for impaired					2,124,419		1,982,075
Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					22,009		23,144
Ending balance: individually evaluated for impaired					3,474		1,818
Ending balance: collectively evaluated for impaired					18,535		21,326
Total					1,116,204		1,154,225
Ending balance: individually evaluated for impaired					12,864		15,995
Ending balance: collectively evaluated for impaired					1,103,340		1,138,230
Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					1,592		1,341
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					1,592		1,341
Total					298,395		259,129
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					298,395		259,129
Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending balance: individually evaluated for impaired					167		102
Ending balance: collectively evaluated for impaired					3,490		2,796
Ending balance: individually evaluated for impaired					4,522		7,142
Ending balance: collectively evaluated for impaired					1,528,816		1,416,280
Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Ending Balance					3,817		3,426
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					3,817		3,426
Total					578,414		548,807
Ending balance: individually evaluated for impaired					0		0
Ending balance: collectively evaluated for impaired					578,414		548,807
Acquired Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	156	172	46	19
Charge-offs	(13)	(26)	(165)	(43)
Recoveries	50	28	136	106
Provision for credit losses	(118)	356	58	448
Ending Balance	156	172	46	19	75	46	530
Total					512,258	400,015
Acquired Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	127	4	6	19
Charge-offs	(9)	(17)	(57)	(26)
Recoveries	25	18	75	45
Provision for credit losses	(96)	1	23	(32)
Ending Balance	127	4	6	19	47	6	6
Total					255,911	220,251
Acquired Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	6	50	29	0
Charge-offs	0	0	0	0
Recoveries	0	0	0	4
Provision for credit losses	2	(6)	(21)	40
Ending Balance	6	50	29	0	8	29	44
Total					207,212	126,911
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	23	118	11	0
Charge-offs	0	0	(93)	0
Recoveries	13	1	31	1
Provision for credit losses	(16)	361	71	479
Ending Balance	23	118	11	0	20	11	480
Total					36,667	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	4	6	5
Provision for credit losses	0	(4)	(6)	(5)
Ending Balance	0	0	0	0	0	0	0
Total					6,750	5,963
Acquired Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	0	0	0	0
Charge-offs	(4)	(9)	(15)	(17)
Recoveries	12	5	24	51
Provision for credit losses	(8)	4	(9)	(34)
Ending Balance	0	0	0	0	0	0	0
Total					5,718	6,248
Originated Loans [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	51,158	47,895	48,252	50,166
Charge-offs	(3,926)	(1,893)	(10,124)	(10,010)
Recoveries	360	786	1,569	5,104
Provision for credit losses	3,079	858	10,974	2,386
Ending Balance	51,158	47,895	48,252	50,166	50,671	48,252	47,646
Total					5,150,524	5,007,361
Originated Loans [Member] | Residential real estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,556	2,116	2,753	2,492
Charge-offs	(268)	(344)	(949)	(954)
Recoveries	26	67	222	259
Provision for credit losses	296	1,053	1,584	1,095
Ending Balance	3,556	2,116	2,753	2,492	3,610	2,753	2,892
Total					1,277,427	1,206,119
Originated Loans [Member] | Commercial Real Estate [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	17,731	7,307	17,328	6,619
Charge-offs	0	(35)	(2,411)	(95)
Recoveries	36	60	123	206
Provision for credit losses	1,896	9,991	4,623	10,593
Ending Balance	17,731	7,307	17,328	6,619	19,663	17,328	17,323
Total					1,929,219	1,892,185
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	25,082	33,372	23,418	35,974
Charge-offs	(2,582)	(499)	(3,443)	(5,776)
Recoveries	53	183	671	3,819
Provision for credit losses	(564)	(10,392)	1,343	(11,353)
Ending Balance	25,082	33,372	23,418	35,974	21,989	23,418	22,664
Total					1,079,537	1,122,741
Originated Loans [Member] | Real estate construction [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	1,262	768	1,349	577
Charge-offs	0	0	0	0
Recoveries	92	369	93	465
Provision for credit losses	238	204	150	299
Ending Balance	1,262	768	1,349	577	1,592	1,349	1,341
Total					291,645	242,905
Originated Loans [Member] | Loans to individuals [Member]
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	3,527	4,332	3,404	4,504
Charge-offs	(1,076)	(1,015)	(3,321)	(3,185)
Recoveries	153	107	460	355
Provision for credit losses	1,213	2	3,274	1,752
Ending Balance	$ 3,527	$ 4,332	$ 3,404	$ 4,504	3,817	3,404	$ 3,426
Total					$ 572,696	$ 543,411