Loans and Leases - Allowance for Credit Losses, Loans and Leases Roll Forward (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				$ 35,331				$ 29,364	$ 35,331	$ 29,364	$ 21,555
Allowance for credit losses, loans and leases	$ 83,044			35,331	$ 35,331			29,364	35,331	29,364	29,364	$ 35,331
(Reversal of provision) provision for credit losses	(8,721)	$ 3,935	$ 23,737	21,843	2,225	$ 1,533	$ 2,073	2,680	39,439	8,518	20,310
Charge-offs									(5,899)	(3,463)	(15,651)
Recoveries									1,251	912	3,150
Ending balance	83,044				35,331				83,044	35,331	29,364
Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												12,922
Commercial, Financial and Agricultural [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				8,759				7,983	8,759	7,983	6,742
Allowance for credit losses, loans and leases	13,584			8,759	8,759			7,983	13,584	7,983	7,983	8,759
(Reversal of provision) provision for credit losses									680	2,374	13,756
Charge-offs									(1,884)	(1,965)	(14,655)
Recoveries									745	367	2,140
Ending balance	13,584				8,759				13,584	8,759	7,983
Commercial, Financial and Agricultural [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												5,284
Real Estate-Commercial [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				15,750					15,750
Allowance for credit losses, loans and leases	52,230			15,750	15,750				52,230	15,750		15,750
(Reversal of provision) provision for credit losses									33,090
Charge-offs									(2,853)
Recoveries									35
Ending balance	52,230				15,750				52,230	15,750
Real Estate-Commercial [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												6,208
Real Estate-Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				2,446					2,446
Allowance for credit losses, loans and leases	3,298			2,446	2,446				3,298	2,446		2,446
(Reversal of provision) provision for credit losses									823
Charge-offs									0
Recoveries									0
Ending balance	3,298				2,446				3,298	2,446
Real Estate-Construction [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												29
Real Estate-Commercial and Construction [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				18,196				13,903	18,196	13,903	9,839
Allowance for credit losses, loans and leases				18,196	18,196			13,903	18,196	18,196	9,839
(Reversal of provision) provision for credit losses										4,602	3,771
Charge-offs										(402)	(40)
Recoveries										93	333
Ending balance					18,196					18,196	13,903
Real Estate-Residential Secured for Business Purpose [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				2,622				2,236	2,622	2,236	1,661
Allowance for credit losses, loans and leases	7,317			2,622	2,622			2,236	2,622	2,622	1,661	2,622
(Reversal of provision) provision for credit losses									2,306	396	326
Charge-offs									(188)	(122)	(31)
Recoveries									75	112	280
Ending balance	7,317				2,622				7,317	2,622	2,236
Real Estate-Residential Secured for Business Purpose [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												2,502
Real Estate-Residential Secured for Personal Purpose [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				2,713					2,713
Allowance for credit losses, loans and leases	3,055			2,713	2,713				3,055	2,713		2,713
(Reversal of provision) provision for credit losses									1,229
Charge-offs									(181)
Recoveries									0
Ending balance	3,055				2,713				3,055	2,713
Real Estate-Residential Secured for Personal Purpose [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												(706)
Real Estate-Home Equity Secured for Personal Purpose [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				1,076					1,076
Allowance for credit losses, loans and leases	1,176			1,076	1,076				1,176	1,076		1,076
(Reversal of provision) provision for credit losses									449
Charge-offs									0
Recoveries									15
Ending balance	1,176				1,076				1,176	1,076
Real Estate-Home Equity Secured for Personal Purpose [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												(364)
Real Estate-Residential and Home Equity Secured for Personal Purpose [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				3,789				3,199	3,789	3,199	1,754
Allowance for credit losses, loans and leases				3,789	3,789			3,199	3,789	3,789	3,199
(Reversal of provision) provision for credit losses										781	1,367
Charge-offs										(212)	0
Recoveries										21	78
Ending balance					3,789					3,789	3,199
Loans to Individuals [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				470				484	470	484	373
Allowance for credit losses, loans and leases	533			470	470			484	470	470	373	470
(Reversal of provision) provision for credit losses									146	246	376
Charge-offs									(267)	(335)	(353)
Recoveries									80	75	88
Ending balance	533				470				533	470	484
Loans to Individuals [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												104
Lease Financings [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				1,311				1,288	1,311	1,288	1,132
Allowance for credit losses, loans and leases	1,701			1,311	1,311			1,288	1,311	1,311	1,132	1,311
(Reversal of provision) provision for credit losses									750	206	497
Charge-offs									(526)	(427)	(572)
Recoveries									301	244	231
Ending balance	1,701				1,311				1,701	1,311	1,288
Lease Financings [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												(135)
Unallocated [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Beginning balance				184				271	184	271	54
Allowance for credit losses, loans and leases	150			$ 184	184			$ 271	150	271	271	184
(Reversal of provision) provision for credit losses									(34)	(87)	217
Ending balance	$ 150				$ 184				$ 150	$ 184	$ 271
Unallocated [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Allowance for Loan and Lease Losses [Roll Forward]
Allowance for credit losses, loans and leases												$ 0