Loans and Leases - Reserve for Loan and Lease Losses and Recorded Investment in Loans and Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	$ 1,415	$ 131
Ending balance: collectively evaluated for impairment	27,901	21,424
Ending balance: acquired non-credit impaired loans evaluated for impairment	48
Total ending balance	29,364	21,555	$ 17,499	$ 17,628
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	26,581	29,732
Ending balance: collectively evaluated for impairment	3,600,838	3,049,597
Loans measured at fair value	1,779	1,958
Acquired non-credit impaired loans	376,681	537,197
Acquired credit impaired loans	695	1,583
Total Loans and Leases Held for Investment	4,006,574	3,620,067
Commercial, Financial and Agricultural [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	413	31
Ending balance: collectively evaluated for impairment	7,570	6,711
Ending balance: acquired non-credit impaired loans evaluated for impairment	0
Total ending balance	7,983	6,742	7,037	6,418
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	3,747	7,079
Ending balance: collectively evaluated for impairment	909,419	826,021
Loans measured at fair value	0	0
Acquired non-credit impaired loans	24,519	62,689
Acquired credit impaired loans	0	422
Total Loans and Leases Held for Investment	937,685	896,211
Real Estate-Commercial and Construction [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	675	99
Ending balance: collectively evaluated for impairment	13,183	9,740
Ending balance: acquired non-credit impaired loans evaluated for impairment	45
Total ending balance	13,903	9,839	7,505	6,572
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	18,321	16,919
Ending balance: collectively evaluated for impairment	1,702,992	1,388,048
Loans measured at fair value	1,779	1,958
Acquired non-credit impaired loans	233,419	310,696
Acquired credit impaired loans	206	356
Total Loans and Leases Held for Investment	1,956,717	1,717,977
Real Estate-Residential Secured for Business Purpose [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0	1
Ending balance: collectively evaluated for impairment	2,233	1,660
Ending balance: acquired non-credit impaired loans evaluated for impairment	3
Total ending balance	2,236	1,661	774	763
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	1,478	3,465
Ending balance: collectively evaluated for impairment	300,915	247,335
Loans measured at fair value	0	0
Acquired non-credit impaired loans	59,977	90,582
Acquired credit impaired loans	426	585
Total Loans and Leases Held for Investment	362,796	341,967
Real Estate-Residential and Home Equity Secured for Personal Purpose [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	327	0
Ending balance: collectively evaluated for impairment	2,872	1,754
Ending balance: acquired non-credit impaired loans evaluated for impairment	0
Total ending balance	3,199	1,754	993	1,575
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	3,035	1,019
Ending balance: collectively evaluated for impairment	512,939	431,519
Loans measured at fair value	0	0
Acquired non-credit impaired loans	58,624	73,086
Acquired credit impaired loans	63	220
Total Loans and Leases Held for Investment	574,661	505,844
Loans to Individuals [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	484	373
Ending balance: acquired non-credit impaired loans evaluated for impairment	0
Total ending balance	484	373	364	346
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	32,617	28,156
Loans measured at fair value	0	0
Acquired non-credit impaired loans	142	144
Acquired credit impaired loans	0	0
Total Loans and Leases Held for Investment	32,759	28,300
Lease Financings [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,288	1,132
Ending balance: acquired non-credit impaired loans evaluated for impairment	0
Total ending balance	1,288	1,132	788	1,042
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	0	1,250
Ending balance: collectively evaluated for impairment	141,956	128,518
Loans measured at fair value	0	0
Acquired non-credit impaired loans	0	0
Acquired credit impaired loans	0	0
Total Loans and Leases Held for Investment	141,956	129,768
Unallocated [Member]
Reserve for loan and lease losses:
Ending balance: collectively evaluated for impairment	271	54
Ending balance: acquired non-credit impaired loans evaluated for impairment	0
Total ending balance	$ 271	$ 54	$ 38	$ 912