Borrowings - Narrative (Details) - USD ($)		3 Months Ended								12 Months Ended
	Mar. 22, 2013	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Ratio of line of credit to eligible assets		35.00%								35.00%
Line of credit		$ 147,700,000.0								$ 147,700,000.0
Ratio of line of credit to total assets		10.00%								10.00%
Ratio of eligible assets to borrowing capacity		35.00%								35.00%
Additional borrowing capacity		$ 531,700,000								$ 531,700,000
Long-term FHLB advances		7,400,000				$ 4,300,000				7,400,000	$ 4,300,000
Term of FHLB advances	18 years		20 years					20 years
Repayment term of FHLB advance	30 years		30 years					30 years
Fixed rate of FHLB advance	3.12%		3.25%					2.61%
Loans pledged as collateral		0				0				0	0
FHLB discount window advances outstanding		0				0				0	0
Interest paid (less than)										$ 2,470,000	2,553,000	$ 2,833,000
Alaska limit on borrowing, percent of total assets										15.00%
Alaska limit on borrowing, total assets		225,800,000.0				227,400,000.0				$ 225,800,000.0	227,400,000.0
Securities sold under agreements to repurchase		$ 27,746,000				$ 27,607,000				$ 27,746,000	$ 27,607,000
Percentage paid on securities sold under agreements to repurchase		0.14%				0.11%				0.14%	0.11%
Average balance outstanding		$ 29,000,000				$ 27,200,000				$ 29,000,000	$ 27,200,000
Maximum amount outstanding at any month end										32,600,000	30,500,000
Interest expense on deposits, borrowings and junior subordinated debentures		$ 548,000	$ 602,000	$ 648,000	$ 632,000	$ 631,000	$ 647,000	$ 639,000	$ 644,000	$ 2,430,000	$ 2,561,000	$ 2,871,000
Weighted average interest rate		3.23%				2.82%				3.23%	2.82%	2.52%
FHLB Community Investment Program Advance
Debt Instrument [Line Items]
Face amount	$ 2,200,000
FHLB Community Investment Cash Advance Program
Debt Instrument [Line Items]
Face amount			$ 3,100,000					$ 2,300,000
Note Agreement With Federal Reserve Bank
Debt Instrument [Line Items]
Loans pledged as collateral		$ 73,600,000								$ 73,600,000
Interest paid (less than)										1,000	$ 1,000
Discount Window
Debt Instrument [Line Items]
Loans pledged as collateral		$ 43,900,000								43,900,000
FHLB and Secured Debt
Debt Instrument [Line Items]
Interest expense on deposits, borrowings and junior subordinated debentures										$ 207,000	$ 157,000	$ 469,000