Loans - Impaired Loan (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Recorded Investment
Impaired loan, with no related allowance, recorded investment	$ 1,659	$ 2,670
Impaired loan, with related allowance, recorded investment	1,558	2,174
Impaired loan, total, recorded investment	3,217	4,844
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,659	2,680
Impaired loan, with related allowance, unpaid principal balance	1,553	2,174
Impaired loan, total, unpaid principal balance	3,212	4,854
Impaired loan, total, related allowance	167	23
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,834	2,100
Impaired loan, with related allowance, average recorded investment	1,762	2,588
Impaired loan, total, average recorded investment	3,596	4,688
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	123	180
Impaired loan, with related allowance, interest income recognized	55	54
Impaired loan, total, interest income recognized	178	234
Commercial
Recorded Investment
Impaired loan, with no related allowance, recorded investment	4	24
Impaired loan, with related allowance, recorded investment	202	19
Impaired loan, total, recorded investment	206	43
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	4	24
Impaired loan, with related allowance, unpaid principal balance	201	19
Impaired loan, total, unpaid principal balance	205	43
Impaired loan, total, related allowance	154	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	19	12
Impaired loan, with related allowance, average recorded investment	150	78
Impaired loan, total, average recorded investment	169	90
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	1	2
Impaired loan, with related allowance, interest income recognized	16	1
Impaired loan, total, interest income recognized	17	3
Commercial real estate | Construction and land development
Recorded Investment
Impaired loan, with no related allowance, recorded investment	0	158
Impaired loan, with related allowance, recorded investment	37	64
Impaired loan, total, recorded investment	37	222
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	157
Impaired loan, with related allowance, unpaid principal balance	37	65
Impaired loan, total, unpaid principal balance	37	222
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	56	198
Impaired loan, with related allowance, average recorded investment	56	272
Impaired loan, total, average recorded investment	112	470
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	4	16
Impaired loan, with related allowance, interest income recognized	0	10
Impaired loan, total, interest income recognized	4	26
Commercial real estate | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	791	1,916
Impaired loan, with related allowance, recorded investment	34	48
Impaired loan, total, recorded investment	825	1,964
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	789	1,917
Impaired loan, with related allowance, unpaid principal balance	32	48
Impaired loan, total, unpaid principal balance	821	1,965
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,069	1,409
Impaired loan, with related allowance, average recorded investment	126	286
Impaired loan, total, average recorded investment	1,195	1,695
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	66	107
Impaired loan, with related allowance, interest income recognized	11	7
Impaired loan, total, interest income recognized	77	114
Residential | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	717	557
Impaired loan, with related allowance, recorded investment	1,022	1,639
Impaired loan, total, recorded investment	1,739	2,196
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	719	567
Impaired loan, with related allowance, unpaid principal balance	1,022	1,639
Impaired loan, total, unpaid principal balance	1,741	2,206
Impaired loan, total, related allowance	12	22
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	575	318
Impaired loan, with related allowance, average recorded investment	1,174	1,593
Impaired loan, total, average recorded investment	1,749	1,911
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	41	38
Impaired loan, with related allowance, interest income recognized	27	32
Impaired loan, total, interest income recognized	68	70
Residential | Home equity
Recorded Investment
Impaired loan, with no related allowance, recorded investment	142	6
Impaired loan, with related allowance, recorded investment	263	386
Impaired loan, total, recorded investment	405	392
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	142	6
Impaired loan, with related allowance, unpaid principal balance	261	385
Impaired loan, total, unpaid principal balance	403	391
Impaired loan, total, related allowance	1	1
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	109	153
Impaired loan, with related allowance, average recorded investment	251	345
Impaired loan, total, average recorded investment	360	498
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	10	16
Impaired loan, with related allowance, interest income recognized	1	4
Impaired loan, total, interest income recognized	11	20
Consumer
Recorded Investment
Impaired loan, with no related allowance, recorded investment	5	9
Impaired loan, with related allowance, recorded investment	0	18
Impaired loan, total, recorded investment	5	27
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	5	9
Impaired loan, with related allowance, unpaid principal balance	0	18
Impaired loan, total, unpaid principal balance	5	27
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	6	10
Impaired loan, with related allowance, average recorded investment	5	14
Impaired loan, total, average recorded investment	11	24
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	1	1
Impaired loan, with related allowance, interest income recognized	0	0
Impaired loan, total, interest income recognized	$ 1	$ 1