Loans and Credit Quality - Narrative (Details)		9 Months Ended	12 Months Ended
	Apr. 01, 2014 USD ($) purchased_loan	Sep. 30, 2018 USD ($) financing_commitment contract	Dec. 31, 2017 USD ($) contract
Receivables [Abstract]
Net nonaccrual loans		$ 16,449,000	$ 21,159,000
Nonperforming loans minimum threshold for individual impairment evaluation		50,000
Recorded investment in loans considered to be impaired		35,434,000	31,967,000
Business Acquisition [Line Items]
Total loans classified as troubled debt restructurings		$ 17,862,000	$ 23,800,000
Number of commitments on TDR | financing_commitment		0
Charge offs on loans later classified as TDRs		$ 965,000
TDR with specific impairment | contract		2	2
TDR that subsequently defaulted | contract		3	0
Alaska Pacific Bancshares
Business Acquisition [Line Items]
Purchased impaired loans | purchased_loan	18
Accretable yield on purchased loans	$ 0
Fair value of purchased credit impaired loans	$ 3,900,000	$ 792,000