Loans - Impaired Loan Balances By Portfolio Segment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Recorded Investment
Impaired loan, with no related allowance, recorded investment	$ 1,725	$ 2,670
Impaired loan, with related allowance, recorded investment	1,927	2,174
Impaired loan, total, recorded investment	3,652	4,844
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,735	2,680
Impaired loan, with related allowance, unpaid principal balance	1,926	2,174
Impaired loan, total, unpaid principal balance	3,661	4,854
Impaired loan, total, related allowance	22	23
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	2,044	2,100
Impaired loan, with related allowance, average recorded investment	2,479	2,588
Impaired loan, total, average recorded investment	4,523	4,688
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	28	180
Impaired loan, with related allowance, interest income recognized	10	54
Impaired loan, total, interest income recognized	38	234
Commercial
Recorded Investment
Impaired loan, with no related allowance, recorded investment	41	24
Impaired loan, with related allowance, recorded investment	77	19
Impaired loan, total, recorded investment	118	43
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	40	24
Impaired loan, with related allowance, unpaid principal balance	77	19
Impaired loan, total, unpaid principal balance	117	43
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	17	12
Impaired loan, with related allowance, average recorded investment	78	78
Impaired loan, total, average recorded investment	95	90
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	1	2
Impaired loan, with related allowance, interest income recognized	1	1
Impaired loan, total, interest income recognized	2	3
Commercial real estate | Construction and land development
Recorded Investment
Impaired loan, with no related allowance, recorded investment	152	158
Impaired loan, with related allowance, recorded investment	68	64
Impaired loan, total, recorded investment	220	222
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	152	157
Impaired loan, with related allowance, unpaid principal balance	68	65
Impaired loan, total, unpaid principal balance	220	222
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	183	198
Impaired loan, with related allowance, average recorded investment	238	272
Impaired loan, total, average recorded investment	421	470
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	2	16
Impaired loan, with related allowance, interest income recognized	0	10
Impaired loan, total, interest income recognized	2	26
Commercial real estate | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	1,027	1,916
Impaired loan, with related allowance, recorded investment	43	48
Impaired loan, total, recorded investment	1,070	1,964
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	1,025	1,917
Impaired loan, with related allowance, unpaid principal balance	43	48
Impaired loan, total, unpaid principal balance	1,068	1,965
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	1,345	1,409
Impaired loan, with related allowance, average recorded investment	245	286
Impaired loan, total, average recorded investment	1,590	1,695
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	16	107
Impaired loan, with related allowance, interest income recognized	0	7
Impaired loan, total, interest income recognized	16	114
Residential | Real estate
Recorded Investment
Impaired loan, with no related allowance, recorded investment	491	557
Impaired loan, with related allowance, recorded investment	1,396	1,639
Impaired loan, total, recorded investment	1,887	2,196
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	504	567
Impaired loan, with related allowance, unpaid principal balance	1,396	1,639
Impaired loan, total, unpaid principal balance	1,900	2,206
Impaired loan, total, related allowance	20	22
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	361	318
Impaired loan, with related allowance, average recorded investment	1,564	1,593
Impaired loan, total, average recorded investment	1,925	1,911
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	9	38
Impaired loan, with related allowance, interest income recognized	8	32
Impaired loan, total, interest income recognized	17	70
Residential | Home equity
Recorded Investment
Impaired loan, with no related allowance, recorded investment	6	6
Impaired loan, with related allowance, recorded investment	333	386
Impaired loan, total, recorded investment	339	392
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	6	6
Impaired loan, with related allowance, unpaid principal balance	332	385
Impaired loan, total, unpaid principal balance	338	391
Impaired loan, total, related allowance	2	1
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	128	153
Impaired loan, with related allowance, average recorded investment	340	345
Impaired loan, total, average recorded investment	468	498
Interest Income Recognized
Impaired loan, with no related allowance, interest income recognized	0	16
Impaired loan, with related allowance, interest income recognized	1	4
Impaired loan, total, interest income recognized	1	20
Consumer
Recorded Investment
Impaired loan, with no related allowance, recorded investment	8	9
Impaired loan, with related allowance, recorded investment	10	18
Impaired loan, total, recorded investment	18	27
Unpaid Principal Balance
Impaired loan, with no related allowance, unpaid principal balance	8	9
Impaired loan, with related allowance, unpaid principal balance	10	18
Impaired loan, total, unpaid principal balance	18	27
Impaired loan, total, related allowance	0	0
Average Recorded Investment
Impaired loan, with no related allowance, average recorded investment	10	10
Impaired loan, with related allowance, average recorded investment	14	14
Impaired loan, total, average recorded investment	24	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