Loans and Leases - Reserve for Loan and Lease Losses and Recorded Investment in Loans and Leases (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	$ 131	$ 235
Ending balance: collectively evaluated for impairment	21,424	17,264
Total ending balance	21,555	17,499	$ 17,628	$ 20,662
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	29,732	43,915
Ending balance: collectively evaluated for impairment	3,049,597	2,346,917
Loans measured at fair value	1,958	2,138
Acquired non-credit impaired loans	537,197	885,564
Acquired credit impaired loans	1,583	7,352
Total Loans and Leases Held for Investment	3,620,067	3,285,886
Commercial, Financial and Agricultural [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	31	19
Ending balance: collectively evaluated for impairment	6,711	7,018
Total ending balance	6,742	7,037	6,418	6,920
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	7,079	11,077
Ending balance: collectively evaluated for impairment	826,021	652,144
Loans measured at fair value	0	0
Acquired non-credit impaired loans	62,689	159,456
Acquired credit impaired loans	422	589
Total Loans and Leases Held for Investment	896,211	823,266
Real Estate-Commercial and Construction [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	99	25
Ending balance: collectively evaluated for impairment	9,740	7,480
Total ending balance	9,839	7,505	6,572	8,943
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	16,919	25,066
Ending balance: collectively evaluated for impairment	1,388,048	1,027,406
Loans measured at fair value	1,958	2,138
Acquired non-credit impaired loans	310,696	489,473
Acquired credit impaired loans	356	5,710
Total Loans and Leases Held for Investment	1,717,977	1,549,793
Real Estate-Residential Secured for Business Purpose [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	1	191
Ending balance: collectively evaluated for impairment	1,660	583
Total ending balance	1,661	774	763	763
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	3,465	6,687
Ending balance: collectively evaluated for impairment	247,335	145,244
Loans measured at fair value	0	0
Acquired non-credit impaired loans	90,582	141,353
Acquired credit impaired loans	585	784
Total Loans and Leases Held for Investment	341,967	294,068
Real Estate-Residential and Home Equity Secured for Personal Purpose [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,754	993
Total ending balance	1,754	993	1,575	1,124
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	1,019	1,085
Ending balance: collectively evaluated for impairment	431,519	357,274
Loans measured at fair value	0	0
Acquired non-credit impaired loans	73,086	95,019
Acquired credit impaired loans	220	269
Total Loans and Leases Held for Investment	505,844	453,647
Loans to Individuals [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	373	364
Total ending balance	373	364	346	360
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	28,156	30,110
Loans measured at fair value	0	0
Acquired non-credit impaired loans	144	263
Acquired credit impaired loans	0	0
Total Loans and Leases Held for Investment	28,300	30,373
Lease Financings [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0	0
Ending balance: collectively evaluated for impairment	1,132	788
Total ending balance	1,132	788	1,042	985
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	1,250	0
Ending balance: collectively evaluated for impairment	128,518	134,739
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	129,768	134,739
Unallocated [Member]
Reserve for loan and lease losses:
Ending balance: collectively evaluated for impairment	54	38
Total ending balance	$ 54	$ 38	$ 912	$ 1,567