Commitments and Contingencies (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) item
Financial instruments whose contract amount represents credit risk [Abstract]
Deposits	$ 694,177,000		$ 694,177,000		$ 602,228,000
Loan balances	681,656,000		681,656,000		660,096,000
Interest income	7,345,000	$ 6,331,000	21,256,000	$ 17,924,000
Noninterest Income	2,344,000	1,399,000	6,405,000	3,762,000
Standby Letters of Credit [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Off-balance sheet credit risk	9,171,000		$ 9,171,000		3,480,000
Letters of credit expiry period			12 months
Letters of credit renewal period			1 year
Current liability for guarantees	100,000		$ 100,000		42,000
Home Equity Lines of Credit [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Off-balance sheet credit risk	17,735,000		$ 17,735,000		13,321,000
Loan expiry period			10 years
Unadvanced Construction Commitments [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Off-balance sheet credit risk	70,408,000		$ 70,408,000		74,720,000
Mortgage Loan Commitments [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Off-balance sheet credit risk					$ 595,000
Number of mortgage loan commitments at fixed interest rate | item					2
Fixed rate loan commitments					$ 595,000
Lines of Credit [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Off-balance sheet credit risk	21,396,000		21,396,000		16,612,000
Loans Sold and Serviced with Limited Repurchase Provisions [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Off-balance sheet credit risk	42,449,000		42,449,000		21,409,000
Mortgage loan, held in reserve	82,000		82,000		63,000
Repurchases of loans previously sold			$ 0	0
Loans Sold and Serviced with Limited Repurchase Provisions [Member] | Minimum [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Period of delinquency under repurchase agreement			120 days
Loans Sold and Serviced with Limited Repurchase Provisions [Member] | Maximum [Member]
Financial instruments whose contract amount represents credit risk [Abstract]
Period of delinquency under repurchase agreement			180 days
Business activities with medical-use cannabis customers [Member] | Cannabis Customers
Financial instruments whose contract amount represents credit risk [Abstract]
Deposits	12,800,000		$ 12,800,000		19,200,000
Loan balances	$ 15,300,000		$ 15,300,000		$ 11,900,000
Percentage of deposits in total deposits	1.80%		1.80%		3.20%
Percentage of loans in total loans	2.20%		2.20%		1.80%
Interest income	$ 231,000	0	$ 541,000	0
Noninterest Income	376,000		921,000
Volume of deposits accepted	$ 41,500,000	$ 0	$ 70,800,000	$ 0