Allowance for Loan Losses - Activity in Allowance for Losses Disaggregated on Basis of Impairment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				$ 32,086				$ 31,083	$ 32,086	$ 31,083	$ 29,746
Charge-offs									(3,605)	(4,838)	(6,085)
Recoveries									1,432	1,621	2,491
Provision for loan losses	$ 1,162	$ 1,065	$ 1,750	1,152	$ 1,338	$ 884	$ 1,131	867	5,129	4,220	4,931
Balance at the end of the year	35,042				32,086				35,042	32,086	31,083
Commercial business
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				21,999				22,382	21,999	22,382	22,064
Charge-offs									(1,400)	(2,438)	(4,153)
Recoveries									908	947	1,844
Provision for loan losses									2,204	1,108	2,627
Balance at the end of the year	23,711				21,999				23,711	21,999	22,382
Commercial business | Commercial and industrial
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				9,910				10,968	9,910	10,968	9,972
Charge-offs									(1,250)	(859)	(3,265)
Recoveries									901	792	1,844
Provision for loan losses									1,782	(991)	2,417
Balance at the end of the year	11,343				9,910				11,343	9,910	10,968
Commercial business | Owner-occupied commercial real estate
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				3,992				3,661	3,992	3,661	4,370
Charge-offs									(1)	(1,579)	(538)
Recoveries									7	155	0
Provision for loan losses									900	1,755	(171)
Balance at the end of the year	4,898				3,992				4,898	3,992	3,661
Commercial business | Non-owner occupied commercial real estate
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				8,097				7,753	8,097	7,753	7,722
Charge-offs									(149)	0	(350)
Recoveries									0	0	0
Provision for loan losses									(478)	344	381
Balance at the end of the year	7,470				8,097				7,470	8,097	7,753
One-to-four family residential
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				1,056				1,015	1,056	1,015	1,157
Charge-offs									(45)	(30)	0
Recoveries									0	2	2
Provision for loan losses									192	69	(144)
Balance at the end of the year	1,203				1,056				1,203	1,056	1,015
Real estate construction and land development
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				2,052				2,156	2,052	2,156	1,871
Charge-offs									0	(556)	(154)
Recoveries									11	202	83
Provision for loan losses									131	250	356
Balance at the end of the year	2,194				2,052				2,194	2,052	2,156
Real estate construction and land development | One-to-four family residential
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				862				797	862	797	1,058
Charge-offs									0	(556)	(100)
Recoveries									11	202	83
Provision for loan losses									367	419	(244)
Balance at the end of the year	1,240				862				1,240	862	797
Real estate construction and land development | Five or more family residential and commercial properties
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				1,190				1,359	1,190	1,359	813
Charge-offs									0	0	(54)
Recoveries									0	0	0
Provision for loan losses									(236)	(169)	600
Balance at the end of the year	954				1,190				954	1,190	1,359
Consumer
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				6,081				5,024	6,081	5,024	4,309
Charge-offs									(2,160)	(1,814)	(1,778)
Recoveries									513	470	562
Provision for loan losses									2,147	2,401	1,931
Balance at the end of the year	6,581				6,081				6,581	6,081	5,024
Unallocated
Schedule of allowance for loan losses on the basis of impairment method
Balance at the beginning of the year				$ 898				$ 506	898	506	345
Charge-offs									0	0	0
Recoveries									0	0	0
Provision for loan losses									455	392	161
Balance at the end of the year	$ 1,353				$ 898				$ 1,353	$ 898	$ 506