Loans - Impaired Loan Balances By Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Recorded Investment
Impaired loan, with no related allowance, recorded investment	$ 1,882	$ 1,659
Impaired loan, with related allowance, recorded investment	580	1,558
Impaired loan, total, recorded investment	2,462	3,217
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,882	1,659
Impaired loan, with related allowance, unpaid principal balance	582	1,553
Impaired loan, total, unpaid principal balance	2,464	3,212
Impaired loan, total, related allowance	160	167
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,772	1,834
Impaired loan, with related allowance, average recorded investment	1,069	1,762
Impaired loan, total, average recorded investment	2,841	3,596
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	21	123
Impaired loan, with related allowance, interest income recognized	4	55
Impaired loan, total, interest income recognized	25	178
Commercial
Recorded Investment
Impaired loan, with no related allowance, recorded investment	0	4
Impaired loan, with related allowance, recorded investment	122	202
Impaired loan, total, recorded investment	122	206
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	4
Impaired loan, with related allowance, unpaid principal balance	121	201
Impaired loan, total, unpaid principal balance	121	205
Impaired loan, total, related allowance	60	154
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	3	19
Impaired loan, with related allowance, average recorded investment	162	150
Impaired loan, total, average recorded investment	165	169
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	0	1
Impaired loan, with related allowance, interest income recognized	1	16
Impaired loan, total, interest income recognized	1	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	35	37
Impaired loan, total, recorded investment	35	37
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	0
Impaired loan, with related allowance, unpaid principal balance	36	37
Impaired loan, total, unpaid principal balance	36	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	36	56
Impaired loan, total, average recorded investment	36	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	529	791
Impaired loan, with related allowance, recorded investment	29	34
Impaired loan, total, recorded investment	558	825
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	526	789
Impaired loan, with related allowance, unpaid principal balance	29	32
Impaired loan, total, unpaid principal balance	555	821
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	660	1,069
Impaired loan, with related allowance, average recorded investment	31	126
Impaired loan, total, average recorded investment	691	1,195
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	10	66
Impaired loan, with related allowance, interest income recognized	0	11
Impaired loan, total, interest income recognized	10	77
Residential | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	1,209	717
Impaired loan, with related allowance, recorded investment	151	1,022
Impaired loan, total, recorded investment	1,360	1,739
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,211	719
Impaired loan, with related allowance, unpaid principal balance	151	1,022
Impaired loan, total, unpaid principal balance	1,362	1,741
Impaired loan, total, related allowance	11	12
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	963	575
Impaired loan, with related allowance, average recorded investment	607	1,174
Impaired loan, total, average recorded investment	1,570	1,749
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	8	41
Impaired loan, with related allowance, interest income recognized	2	27
Impaired loan, total, interest income recognized	10	68
Residential | Home equity
Recorded Investment
Impaired loan, with no related allowance, recorded investment	140	142
Impaired loan, with related allowance, recorded investment	243	263
Impaired loan, total, recorded investment	383	405
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	141	142
Impaired loan, with related allowance, unpaid principal balance	245	261
Impaired loan, total, unpaid principal balance	386	403
Impaired loan, total, related allowance	89	1
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	141	109
Impaired loan, with related allowance, average recorded investment	233	251
Impaired loan, total, average recorded investment	374	360
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	3	10
Impaired loan, with related allowance, interest income recognized	1	1
Impaired loan, total, interest income recognized	4	11
Consumer
Recorded Investment
Impaired loan, with no related allowance, recorded investment	4	5
Impaired loan, with related allowance, recorded investment	0	0
Impaired loan, total, recorded investment	4	5
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	4	5
Impaired loan, with related allowance, unpaid principal balance	0	0
Impaired loan, total, unpaid principal balance	4	5
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	5	6
Impaired loan, with related allowance, average recorded investment	0	5
Impaired loan, total, average recorded investment	5	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