Loans - Impaired Loan Balances By Portfolio Segment (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Recorded Investment
Impaired loan, with no related allowance, recorded investment	$ 2,165	$ 2,670
Impaired loan, with related allowance, recorded investment	1,476	2,174
Impaired loan, total, recorded investment	3,641	4,844
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	2,165	2,680
Impaired loan, with related allowance, unpaid principal balance	1,470	2,174
Impaired loan, total, unpaid principal balance	3,635	4,854
Impaired loan, total, related allowance	165	23
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,988	2,100
Impaired loan, with related allowance, average recorded investment	2,267	2,588
Impaired loan, total, average recorded investment	4,255	4,688
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	100	180
Impaired loan, with related allowance, interest income recognized	32	54
Impaired loan, total, interest income recognized	132	234
Commercial
Recorded Investment
Impaired loan, with no related allowance, recorded investment	0	24
Impaired loan, with related allowance, recorded investment	257	19
Impaired loan, total, recorded investment	257	43
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	24
Impaired loan, with related allowance, unpaid principal balance	255	19
Impaired loan, total, unpaid principal balance	255	43
Impaired loan, total, related allowance	155	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	17	12
Impaired loan, with related allowance, average recorded investment	98	78
Impaired loan, total, average recorded investment	115	90
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	1	2
Impaired loan, with related allowance, interest income recognized	9	1
Impaired loan, total, interest income recognized	10	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	59	64
Impaired loan, total, recorded investment	59	222
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	0	157
Impaired loan, with related allowance, unpaid principal balance	59	65
Impaired loan, total, unpaid principal balance	59	222
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	124	198
Impaired loan, with related allowance, average recorded investment	193	272
Impaired loan, total, average recorded investment	317	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	1,053	1,916
Impaired loan, with related allowance, recorded investment	95	48
Impaired loan, total, recorded investment	1,148	1,964
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,051	1,917
Impaired loan, with related allowance, unpaid principal balance	93	48
Impaired loan, total, unpaid principal balance	1,144	1,965
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,274	1,409
Impaired loan, with related allowance, average recorded investment	208	286
Impaired loan, total, average recorded investment	1,482	1,695
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	55	107
Impaired loan, with related allowance, interest income recognized	0	7
Impaired loan, total, interest income recognized	55	114
Residential | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	962	557
Impaired loan, with related allowance, recorded investment	795	1,639
Impaired loan, total, recorded investment	1,757	2,196
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	964	567
Impaired loan, with related allowance, unpaid principal balance	794	1,639
Impaired loan, total, unpaid principal balance	1,758	2,206
Impaired loan, total, related allowance	9	22
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	432	318
Impaired loan, with related allowance, average recorded investment	1,446	1,593
Impaired loan, total, average recorded investment	1,878	1,911
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	32	38
Impaired loan, with related allowance, interest income recognized	22	32
Impaired loan, total, interest income recognized	54	70
Residential | Home equity
Recorded Investment
Impaired loan, with no related allowance, recorded investment	144	6
Impaired loan, with related allowance, recorded investment	269	386
Impaired loan, total, recorded investment	413	392
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	144	6
Impaired loan, with related allowance, unpaid principal balance	268	385
Impaired loan, total, unpaid principal balance	412	391
Impaired loan, total, related allowance	1	1
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	132	153
Impaired loan, with related allowance, average recorded investment	311	345
Impaired loan, total, average recorded investment	443	498
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	8	16
Impaired loan, with related allowance, interest income recognized	1	4
Impaired loan, total, interest income recognized	9	20
Consumer
Recorded Investment
Impaired loan, with no related allowance, recorded investment	6	9
Impaired loan, with related allowance, recorded investment	1	18
Impaired loan, total, recorded investment	7	27
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	6	9
Impaired loan, with related allowance, unpaid principal balance	1	18
Impaired loan, total, unpaid principal balance	7	27
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	9	10
Impaired loan, with related allowance, average recorded investment	11	14
Impaired loan, total, average recorded investment	20	24
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