Loans and Allowance for Credit Losses - Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	$ 47,764	$ 48,298
Charge-offs	(12,300)	(15,100)
Provision for credit losses	14,533	12,531	$ 5,087
Ending Balance	51,637	47,764	48,298
Ending balance: individually evaluated for impaired	2,432	1,631	3,746
Ending balance: collectively evaluated for impaired	49,205	46,133	44,552
Ending balance	6,189,148	5,774,139	5,407,376
Ending balance: individually evaluated for impaired	17,833	21,088	35,151
Ending balance: collectively evaluated for impaired	6,171,315	5,753,051	5,372,225
Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,374	23,429
Ending Balance	20,234	19,374	23,429
Ending balance: individually evaluated for impaired	1,580	928	3,478
Ending balance: collectively evaluated for impaired	18,654	18,446	19,951
Ending balance	1,241,853	1,138,473	1,163,383
Ending balance: individually evaluated for impaired	9,246	11,631	22,450
Ending balance: collectively evaluated for impaired	1,232,607	1,126,842	1,140,933
Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,002	1,349
Ending Balance	2,558	2,002	1,349
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	2,558	2,002	1,349
Ending balance	449,039	358,978	248,868
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	449,039	358,978	248,868
Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	3,969	2,759
Ending Balance	4,093	3,969	2,759
Ending balance: individually evaluated for impaired	1	107	111
Ending balance: collectively evaluated for impaired	4,092	3,862	2,648
Ending balance	1,681,362	1,562,405	1,426,370
Ending balance: individually evaluated for impaired	1,741	3,747	6,698
Ending balance: collectively evaluated for impaired	1,679,621	1,558,658	1,419,672
Commercial real estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	18,386	17,357
Ending Balance	19,768	18,386	17,357
Ending balance: individually evaluated for impaired	851	596	157
Ending balance: collectively evaluated for impaired	18,917	17,790	17,200
Ending balance	2,117,519	2,123,544	2,019,096
Ending balance: individually evaluated for impaired	6,846	5,710	6,003
Ending balance: collectively evaluated for impaired	2,110,673	2,117,834	2,013,093
Loans To Individuals [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,033	3,404
Ending Balance	4,984	4,033	3,404
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	4,984	4,033	3,404
Ending balance	699,375	590,739	549,659
Ending balance: individually evaluated for impaired	0	0	0
Ending balance: collectively evaluated for impaired	699,375	590,739	549,659
Originated Loans [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	47,586	48,252	50,166
Charge-offs	(10,032)	(14,902)	(11,997)
Recoveries	1,449	1,846	5,403
Provision for credit losses	12,582	12,390	4,680
Ending Balance	51,585	47,586	48,252
Ending balance	5,714,373	5,275,296
Originated Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	19,235	23,418	35,974
Charge-offs	(2,667)	(5,201)	(6,176)
Recoveries	245	746	3,900
Provision for credit losses	3,408	272	(10,280)
Ending Balance	20,221	19,235	23,418
Ending balance	1,212,026	1,100,947
Originated Loans [Member] | Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	2,002	1,349	577
Charge-offs	0	0	0
Recoveries	158	135	465
Provision for credit losses	398	518	307
Ending Balance	2,558	2,002	1,349
Ending balance	442,777	353,008
Originated Loans [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	3,934	2,753	2,492
Charge-offs	(986)	(1,217)	(1,261)
Recoveries	246	233	304
Provision for credit losses	897	2,165	1,218
Ending Balance	4,091	3,934	2,753
Ending balance	1,415,808	1,313,645
Originated Loans [Member] | Commercial real estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	18,382	17,328	6,619
Charge-offs	(632)	(3,930)	(340)
Recoveries	189	153	274
Provision for credit losses	1,792	4,831	10,775
Ending Balance	19,731	18,382	17,328
Ending balance	1,958,346	1,922,349
Originated Loans [Member] | Loans To Individuals [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4,033	3,404	4,504
Charge-offs	(5,747)	(4,554)	(4,220)
Recoveries	611	579	460
Provision for credit losses	6,087	4,604	2,660
Ending Balance	4,984	4,033	3,404
Ending balance	685,416	585,347
Acquired Loans [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	178	46	19
Charge-offs	(2,242)	(211)	(512)
Recoveries	165	202	132
Provision for credit losses	1,951	141	407
Ending Balance	52	178	46
Ending balance	474,775	498,843
Acquired Loans [Member] | Commercial, financial, agricultural and other [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	139	11	0
Charge-offs	(726)	(93)	(458)
Recoveries	81	42	1
Provision for credit losses	519	179	468
Ending Balance	13	139	11
Ending balance	29,827	37,526
Acquired Loans [Member] | Real Estate Construction [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0	0
Charge-offs	0	0	0
Recoveries	0	6	5
Provision for credit losses	0	(6)	(5)
Ending Balance	0	0	0
Ending balance	6,262	5,970
Acquired Loans [Member] | Residential Real Estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	35	6	19
Charge-offs	(56)	(96)	(26)
Recoveries	69	128	67
Provision for credit losses	(46)	(3)	(54)
Ending Balance	2	35	6
Ending balance	265,554	248,760
Acquired Loans [Member] | Commercial real estate [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	4	29	0
Charge-offs	(1,376)	0	0
Recoveries	0	0	4
Provision for credit losses	1,409	(25)	25
Ending Balance	37	4	29
Ending balance	159,173	201,195
Acquired Loans [Member] | Loans To Individuals [Member]
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning Balance	0	0	0
Charge-offs	(84)	(22)	(28)
Recoveries	15	26	55
Provision for credit losses	69	(4)	(27)
Ending Balance	0	0	$ 0
Ending balance	$ 13,959	$ 5,392