Loans - Impaired Loan Balances By Portfolio Segment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Recorded Investment
Impaired loan, with no related allowance, recorded investment	$ 1,237	$ 1,659
Impaired loan, with related allowance, recorded investment	1,178	1,558
Impaired loan, total, recorded investment	2,415	3,217
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,233	1,659
Impaired loan, with related allowance, unpaid principal balance	1,176	1,553
Impaired loan, total, unpaid principal balance	2,409	3,212
Impaired loan, total, related allowance	800	167
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,688	1,834
Impaired loan, with related allowance, average recorded investment	1,020	1,762
Impaired loan, total, average recorded investment	2,708	3,596
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	63	123
Impaired loan, with related allowance, interest income recognized	57	55
Impaired loan, total, interest income recognized	120	178
Commercial
Recorded Investment
Impaired loan, with no related allowance, recorded investment	32	4
Impaired loan, with related allowance, recorded investment	1,002	202
Impaired loan, total, recorded investment	1,034	206
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	32	4
Impaired loan, with related allowance, unpaid principal balance	1,000	201
Impaired loan, total, unpaid principal balance	1,032	205
Impaired loan, total, related allowance	791	154
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	48	19
Impaired loan, with related allowance, average recorded investment	427	150
Impaired loan, total, average recorded investment	475	169
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	4	1
Impaired loan, with related allowance, interest income recognized	49	16
Impaired loan, total, interest income recognized	53	17
Commercial real estate | Construction and land development
Recorded Investment
Impaired loan, with no related allowance, recorded investment	0	0
Impaired loan, with related allowance, recorded investment	0	37
Impaired loan, total, recorded investment	0	37
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	0
Impaired loan, with related allowance, unpaid principal balance	0	37
Impaired loan, total, unpaid principal balance	0	37
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	0	56
Impaired loan, with related allowance, average recorded investment	26	56
Impaired loan, total, average recorded investment	26	112
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	0	4
Impaired loan, with related allowance, interest income recognized	0	0
Impaired loan, total, interest income recognized	0	4
Commercial real estate | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	496	791
Impaired loan, with related allowance, recorded investment	0	34
Impaired loan, total, recorded investment	496	825
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	492	789
Impaired loan, with related allowance, unpaid principal balance	0	32
Impaired loan, total, unpaid principal balance	492	821
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	583	1,069
Impaired loan, with related allowance, average recorded investment	23	126
Impaired loan, total, average recorded investment	606	1,195
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	29	66
Impaired loan, with related allowance, interest income recognized	0	11
Impaired loan, total, interest income recognized	29	77
Residential | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	658	717
Impaired loan, with related allowance, recorded investment	176	1,022
Impaired loan, total, recorded investment	834	1,739
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	658	719
Impaired loan, with related allowance, unpaid principal balance	176	1,022
Impaired loan, total, unpaid principal balance	834	1,741
Impaired loan, total, related allowance	9	12
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	932	575
Impaired loan, with related allowance, average recorded investment	384	1,174
Impaired loan, total, average recorded investment	1,316	1,749
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	21	41
Impaired loan, with related allowance, interest income recognized	7	27
Impaired loan, total, interest income recognized	28	68
Residential | Home equity
Recorded Investment
Impaired loan, with no related allowance, recorded investment	51	142
Impaired loan, with related allowance, recorded investment	0	263
Impaired loan, total, recorded investment	51	405
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	51	142
Impaired loan, with related allowance, unpaid principal balance	0	261
Impaired loan, total, unpaid principal balance	51	403
Impaired loan, total, related allowance	0	1
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	123	109
Impaired loan, with related allowance, average recorded investment	160	251
Impaired loan, total, average recorded investment	283	360
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	9	10
Impaired loan, with related allowance, interest income recognized	1	1
Impaired loan, total, interest income recognized	10	11
Consumer
Recorded Investment
Impaired loan, with no related allowance, recorded investment	0	5
Impaired loan, with related allowance, recorded investment	0	0
Impaired loan, total, recorded investment	0	5
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	5
Impaired loan, with related allowance, unpaid principal balance	0	0
Impaired loan, total, unpaid principal balance	0	5
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	2	6
Impaired loan, with related allowance, average recorded investment	0	5
Impaired loan, total, average recorded investment	2	11
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	0	1
Impaired loan, with related allowance, interest income recognized	0	0
Impaired loan, total, interest income recognized	$ 0	$ 1