Fair Value - Assets and Liabilities at Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, carrying value	$ 16,559	$ 19,207	$ 17,600	$ 17,366
Interest bearing deposits in other banks	280	860	3,272
Securities available for sale	303,769	300,172	312,624
Securities to be held to maturity, carrying value	258,690	256,567	240,829
Securities to be held to maturity	255,851	259,655	239,378
Restricted equity securities	11,947	10,358	13,363
Loans held for sale, carrying value	284	386	979
Mortgage servicing rights	2,518	2,321	1,853
Demand deposits	137,674	145,332	130,319
NOW deposits	314,587	318,043	323,919
Money market deposits	108,726	163,898	142,220
Savings deposits	232,458	232,605	222,976
Total deposits	1,428,192	1,418,879	1,346,483
Carrying value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents, carrying value	16,559	19,207	17,600
Interest bearing deposits in other banks	280	860	3,272
Securities available for sale	303,769	300,172	312,624
Securities to be held to maturity, carrying value	258,690	256,567	240,829
Restricted equity securities	11,947	10,358	13,363
Loans held for sale, carrying value	284	386	979
Loans	1,177,045	1,153,410	1,079,368
Mortgage servicing rights, carrying value	1,280	1,268	1,054
Interest rate swap agreements	2,441	1,955	1,886
Accrued interest receivable	7,222	5,867	6,854
Demand deposits	137,674	145,332	130,319
NOW deposits	314,587	318,043	323,919
Money market deposits	108,726	163,898	142,220
Savings deposits	232,458	232,605	222,976
Local certificates of deposit	260,450	223,074	213,296
National certificates of deposit	374,297	335,927	313,753
Total deposits	1,428,192	1,418,879	1,346,483
Repurchase agreements	67,182	70,564	76,342
Federal Home Loan Bank advances	177,047	158,194	150,125
Total borrowed funds	244,229	228,758	226,467
Accrued interest payable	760	642	519
Carrying value | Commercial | Real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	335,275	319,691	299,920
Carrying value | Commercial | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	43,385	37,594	28,254
Carrying value | Commercial | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	173,960	177,956	156,233
Carrying value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	35,440	33,370	28,306
Carrying value | Residential | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	15,814	17,830	13,689
Carrying value | Residential | Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	438,765	431,459	420,057
Carrying value | Home equity line of credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	109,525	110,566	109,058
Carrying value | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	24,881	24,944	23,851
Total Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	16,559	19,207	17,600
Interest bearing deposits in other banks	280	860	3,272
Securities available for sale	303,769	300,172	312,624
Securities to be held to maturity	255,851	259,655	239,378
Restricted equity securities	11,947	10,358	13,363
Loans held for sale	284	386	979
Loans	1,152,071	1,138,727	1,071,721
Mortgage servicing rights	2,518	2,321	1,853
Interest rate swap agreements	2,441	1,955	1,886
Accrued interest receivable	7,222	5,867	6,854
Demand deposits	128,525	139,350	123,696
NOW deposits	286,661	295,775	296,958
Money market deposits	102,196	153,497	130,374
Savings deposits	198,171	203,799	193,142
Local certificates of deposit	255,579	220,734	212,337
National certificates of deposit	374,095	335,775	313,833
Total deposits	1,345,227	1,348,930	1,270,340
Repurchase agreements	64,361	67,976	72,399
Federal Home Loan Bank advances	175,308	156,396	149,358
Total borrowed funds	239,669	224,372	221,757
Accrued interest payable	760	642	519
Total Fair Value | Commercial | Real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	326,030	311,321	295,039
Total Fair Value | Commercial | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	42,189	36,610	27,794
Total Fair Value | Commercial | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	171,045	175,455	154,823
Total Fair Value | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	34,972	33,280	28,737
Total Fair Value | Residential | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	15,587	17,613	13,593
Total Fair Value | Residential | Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	430,023	431,028	420,393
Total Fair Value | Home equity line of credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	108,047	109,012	107,823
Total Fair Value | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	24,178	24,408	23,519
Total Fair Value | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	16,559	19,207	17,600
Interest bearing deposits in other banks	280	860	3,272
Securities available for sale	0	0	0
Securities to be held to maturity	0	0	0
Restricted equity securities	0	0	0
Loans held for sale	0	0	0
Loans	0	0	0
Mortgage servicing rights	0	0	0
Interest rate swap agreements	0	0	0
Accrued interest receivable	0	0	0
Demand deposits	0	0	0
NOW deposits	0	0	0
Money market deposits	0	0	0
Savings deposits	0	0	0
Local certificates of deposit	0	0	0
National certificates of deposit	0	0	0
Total deposits	0	0	0
Repurchase agreements	0	0	0
Federal Home Loan Bank advances	0	0	0
Total borrowed funds	0	0	0
Accrued interest payable	0	0	0
Total Fair Value | Level 1 | Commercial | Real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Commercial | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Commercial | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Residential | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Residential | Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Home equity line of credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 1 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0	0
Interest bearing deposits in other banks	0	0	0
Securities available for sale	303,769	300,172	312,624
Securities to be held to maturity	255,851	259,655	239,378
Restricted equity securities	11,947	10,358	13,363
Loans held for sale	284	386	979
Loans	5,938	6,521	718
Mortgage servicing rights	2,518	2,321	1,853
Interest rate swap agreements	2,441	1,955	1,886
Accrued interest receivable	7,222	5,867	6,854
Demand deposits	128,525	139,350	123,696
NOW deposits	286,661	295,775	296,958
Money market deposits	102,196	153,497	130,374
Savings deposits	198,171	203,799	193,142
Local certificates of deposit	255,579	220,734	212,337
National certificates of deposit	374,095	335,775	313,833
Total deposits	1,345,227	1,348,930	1,270,340
Repurchase agreements	64,361	67,976	72,399
Federal Home Loan Bank advances	175,308	156,396	149,358
Total borrowed funds	239,669	224,372	221,757
Accrued interest payable	760	642	519
Total Fair Value | Level 2 | Commercial | Real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	3	72	424
Total Fair Value | Level 2 | Commercial | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 2 | Commercial | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	5,495	6,018	33
Total Fair Value | Level 2 | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 2 | Residential | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 2 | Residential | Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	334	391	261
Total Fair Value | Level 2 | Home equity line of credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	106	40	0
Total Fair Value | Level 2 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	0	0	0
Total Fair Value | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	0	0	0
Interest bearing deposits in other banks	0	0	0
Securities available for sale	0	0	0
Securities to be held to maturity	0	0	0
Restricted equity securities	0	0	0
Loans held for sale	0	0	0
Loans	1,146,133	1,132,206	1,071,003
Mortgage servicing rights	0	0	0
Interest rate swap agreements	0	0	0
Accrued interest receivable	0	0	0
Demand deposits	0	0	0
NOW deposits	0	0	0
Money market deposits	0	0	0
Savings deposits	0	0	0
Local certificates of deposit	0	0	0
National certificates of deposit	0	0	0
Total deposits	0	0	0
Repurchase agreements	0	0	0
Federal Home Loan Bank advances	0	0	0
Total borrowed funds	0	0	0
Accrued interest payable	0	0	0
Total Fair Value | Level 3 | Commercial | Real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	326,027	311,249	294,615
Total Fair Value | Level 3 | Commercial | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	42,189	36,610	27,794
Total Fair Value | Level 3 | Commercial | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	165,550	169,437	154,790
Total Fair Value | Level 3 | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	34,972	33,280	28,737
Total Fair Value | Level 3 | Residential | Construction
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	15,587	17,613	13,593
Total Fair Value | Level 3 | Residential | Term
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	429,689	430,637	420,132
Total Fair Value | Level 3 | Home equity line of credit
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	107,941	108,972	107,823
Total Fair Value | Level 3 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	$ 24,178	$ 24,408	$ 23,519