Loans - Impaired Loans by Class (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	$ 18,603	$ 20,235	$ 18,023
Recorded investment, with related allowance	13,112	7,617	13,369
Recorded Investment	31,715	27,852	31,392
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	20,140	22,308	19,775
Unpaid principal balance with related allowance	13,597	7,990	13,698
Unpaid Principal Balance	33,737	30,298	33,473
Related Allowance	2,206	768	1,812
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	18,012	19,817	19,227
Average recorded investment, with related allowance	13,109	7,732	9,881
Average Recorded Investment	31,121	27,549	29,108
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	146	211	553
Recognized interest income, with related allowance	63	82	231
Recognized Interest Income	209	293	784
Commercial | Real estate
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	5,054	5,949	3,791
Recorded investment, with related allowance	3,897	4,722	3,999
Recorded Investment	8,951	10,671	7,790
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	5,269	6,450	3,996
Unpaid principal balance with related allowance	4,002	4,908	4,116
Unpaid Principal Balance	9,271	11,358	8,112
Related Allowance	254	351	224
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	4,163	5,444	5,124
Average recorded investment, with related allowance	3,889	4,741	4,460
Average Recorded Investment	8,052	10,185	9,584
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	54	51	164
Recognized interest income, with related allowance	39	46	152
Recognized Interest Income	93	97	316
Commercial | Construction
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	741	0	741
Recorded investment, with related allowance	0	763	0
Recorded Investment	741	763	741
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	741	0	741
Unpaid principal balance with related allowance	0	763	0
Unpaid Principal Balance	741	763	741
Related Allowance	0	101	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	741	0	62
Average recorded investment, with related allowance	0	763	699
Average Recorded Investment	741	763	761
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	10	0	38
Recognized interest income, with related allowance	0	9	0
Recognized Interest Income	10	9	38
Commercial | Other
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	2,281	1,476	2,591
Recorded investment, with related allowance	7,159	234	7,327
Recorded Investment	9,440	1,710	9,918
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	2,360	1,636	2,671
Unpaid principal balance with related allowance	7,324	272	7,371
Unpaid Principal Balance	9,684	1,908	10,042
Related Allowance	1,664	39	1,309
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	2,302	1,595	1,908
Average recorded investment, with related allowance	7,182	125	2,584
Average Recorded Investment	9,484	1,720	4,492
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	6	22	36
Recognized interest income, with related allowance	0	4	0
Recognized Interest Income	6	26	36
Municipal
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	0	0	0
Recorded investment, with related allowance	0	0	0
Recorded Investment	0	0	0
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	0	0	0
Unpaid principal balance with related allowance	0	0	0
Unpaid Principal Balance	0	0	0
Related Allowance	0	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	0	0	0
Average recorded investment, with related allowance	0	0	0
Average Recorded Investment	0	0	0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	0	0	0
Recognized interest income, with related allowance	0	0	0
Recognized Interest Income	0	0	0
Residential | Construction
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	0	0	0
Recorded investment, with related allowance	0	0	0
Recorded Investment	0	0	0
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	0	0	0
Unpaid principal balance with related allowance	0	0	0
Unpaid Principal Balance	0	0	0
Related Allowance	0	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	0	0	0
Average recorded investment, with related allowance	0	0	0
Average Recorded Investment	0	0	0
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	0	0	0
Recognized interest income, with related allowance	0	0	0
Recognized Interest Income	0	0	0
Residential | Term
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	9,594	11,388	9,769
Recorded investment, with related allowance	1,934	1,872	1,979
Recorded Investment	11,528	13,260	11,748
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	10,733	12,470	10,909
Unpaid principal balance with related allowance	2,146	2,020	2,144
Unpaid Principal Balance	12,879	14,490	13,053
Related Allowance	272	251	255
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	9,700	11,402	10,770
Average recorded investment, with related allowance	1,968	2,077	2,106
Average Recorded Investment	11,668	13,479	12,876
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	71	129	297
Recognized interest income, with related allowance	24	23	79
Recognized Interest Income	95	152	376
Home equity line of credit
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	917	1,422	1,115
Recorded investment, with related allowance	122	26	64
Recorded Investment	1,039	1,448	1,179
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	1,008	1,752	1,429
Unpaid principal balance with related allowance	125	27	67
Unpaid Principal Balance	1,133	1,779	1,496
Related Allowance	16	26	24
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	1,090	1,376	1,351
Average recorded investment, with related allowance	70	26	32
Average Recorded Investment	1,160	1,402	1,383
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	5	9	18
Recognized interest income, with related allowance	0	0	0
Recognized Interest Income	5	9	18
Consumer
Impaired Financing Receivable, Recorded Investment [Abstract]
Recorded investment, with no related allowance	16	0	16
Recorded investment, with related allowance	0	0	0
Recorded Investment	16	0	16
Impaired Financing Receivable, Unpaid Principal Balance [Abstract]
Unpaid principal balance, with no related allowance	29	0	29
Unpaid principal balance with related allowance	0	0	0
Unpaid Principal Balance	29	0	29
Related Allowance	0	0	0
Impaired Financing Receivable, Average Recorded Investment [Abstract]
Average recorded investment, with no related allowance	16	0	12
Average recorded investment, with related allowance	0	0	0
Average Recorded Investment	16	0	12
Impaired Financing Receivable, Interest Income, Cash Basis Method [Abstract]
Recognized interest income, with no related allowance	0	0	0
Recognized interest income, with related allowance	0	0	0
Recognized Interest Income	$ 0	$ 0	$ 0