Loans and Leases - Reserve for Loan and Lease Losses and Recorded Investment in Loans and Leases (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	$ 1,048			$ 88
Ending balance: collectively evaluated for impairment	26,198			20,455
Ending balance: acquired credit impaired loans evaluated for impairment	125
Total ending balance	27,371	$ 25,652	$ 21,555	20,543	$ 20,910	$ 17,499
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	29,972			31,810
Ending balance: collectively evaluated for impairment	3,420,808			2,857,157
Loans measured at fair value	1,801			2,014
Acquired non-credit impaired loans	412,688			594,561
Acquired credit impaired loans	900			1,622
Total Loans and Leases Held for Investment	3,866,169		3,620,067	3,487,164
Commercial, Financial and Agricultural [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	211			15
Ending balance: collectively evaluated for impairment	6,978			6,601
Ending balance: acquired credit impaired loans evaluated for impairment	0
Total ending balance	7,189	7,258	6,742	6,616	8,313	7,037
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	4,889			7,883
Ending balance: collectively evaluated for impairment	862,747			774,886
Loans measured at fair value	0			0
Acquired non-credit impaired loans	26,504			75,983
Acquired credit impaired loans	182			465
Total Loans and Leases Held for Investment	894,322		896,211	859,217
Real Estate-Commercial and Construction [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	645			40
Ending balance: collectively evaluated for impairment	12,504			9,216
Ending balance: acquired credit impaired loans evaluated for impairment	85
Total ending balance	13,234	12,327	9,839	9,256	8,468	7,505
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	18,970			17,274
Ending balance: collectively evaluated for impairment	1,610,805			1,307,585
Loans measured at fair value	1,801			2,014
Acquired non-credit impaired loans	257,998			344,818
Acquired credit impaired loans	206			356
Total Loans and Leases Held for Investment	1,889,780			1,672,047
Real Estate-Residential Secured for Business Purpose [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0			33
Ending balance: collectively evaluated for impairment	2,014			1,272
Ending balance: acquired credit impaired loans evaluated for impairment	40
Total ending balance	2,054	2,004	1,661	1,305	1,129	774
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	1,588			4,471
Ending balance: collectively evaluated for impairment	278,212			209,340
Loans measured at fair value	0			0
Acquired non-credit impaired loans	66,083			95,625
Acquired credit impaired loans	448			584
Total Loans and Leases Held for Investment	346,331		341,967	310,020
Real Estate-Residential and Home Equity Secured for Personal Purpose [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	192			0
Ending balance: collectively evaluated for impairment	2,836			1,715
Ending balance: acquired credit impaired loans evaluated for impairment	0
Total ending balance	3,028	2,494	1,754	1,715	974	993
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	3,275			932
Ending balance: collectively evaluated for impairment	502,190			415,161
Loans measured at fair value	0			0
Acquired non-credit impaired loans	61,961			77,991
Acquired credit impaired loans	64			217
Total Loans and Leases Held for Investment	567,490			494,301
Loans to Individuals [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0			0
Ending balance: collectively evaluated for impairment	472			354
Ending balance: acquired credit impaired loans evaluated for impairment	0
Total ending balance	472	447	373	354	329	364
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	0			0
Ending balance: collectively evaluated for impairment	32,096			27,297
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	32,238		28,300	27,441
Lease Financings [Member]
Reserve for loan and lease losses:
Ending balance: individually evaluated for impairment	0			0
Ending balance: collectively evaluated for impairment	1,137			1,290
Ending balance: acquired credit impaired loans evaluated for impairment	0
Total ending balance	1,137	1,071	1,132	1,290	1,660	788
Loans and leases held for investment:
Ending balance: individually evaluated for impairment	1,250			1,250
Ending balance: collectively evaluated for impairment	134,758			122,888
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	136,008		129,768	124,138
Unallocated Financing Receivables [Member]
Reserve for loan and lease losses:
Ending balance: collectively evaluated for impairment	257			7
Ending balance: acquired credit impaired loans evaluated for impairment	0
Total ending balance	$ 257	$ 51	$ 54	$ 7	$ 37	$ 38