Borrowings - Narrative (Details) - USD ($)		3 Months Ended										12 Months Ended
	Mar. 22, 2013	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2016	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Ratio of line of credit to eligible assets		45.00%										45.00%
Line of credit		$ 143,500,000.0										$ 143,500,000.0
Ratio of line of credit to total assets		10.00%										10.00%
Ratio of eligible assets to borrowing capacity		45.00%										45.00%
Additional borrowing capacity		$ 734,000,000										$ 734,000,000
Long-term FHLB advances		8,900,000				$ 7,200,000						8,900,000	$ 7,200,000
Term of FHLB advances	18 years		20 years							20 years	20 years
Repayment term of FHLB advance	30 years		30 years							30 years	30 years
Fixed rate of FHLB advance	3.12%		2.69%							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)												$ 5,640,000	2,971,000	$ 2,470,000
Alaska limit on borrowing, percent of total assets												15.00%
Alaska limit on borrowing, total assets		244,700,000				222,600,000						$ 244,700,000	222,600,000
Securities sold under agreements to repurchase		$ 0				$ 34,278,000						$ 0	$ 34,278,000
Percentage paid on securities sold under agreements to repurchase		0.00%				0.17%						0.00%	0.17%
Average balance outstanding		$ 15,200,000				$ 29,900,000						$ 15,200,000	$ 29,900,000
Maximum amount outstanding at any month end												36,600,000	36,500,000
Interest expense on deposits, borrowings and junior subordinated debentures		$ 1,652,000	$ 1,531,000	$ 1,349,000	$ 1,109,000	$ 1,070,000	$ 761,000	$ 606,000	$ 532,000			$ 5,641,000	$ 2,969,000	$ 2,430,000
Weighted average interest rate		3.39%				3.39%						3.39%	3.39%	3.23%
FHLB Community Investment Program Advance, First Advance
Debt Instrument [Line Items]
Face amount	$ 1,900,000.0
FHLB Community Investment Cash Advance Program, Second Advance
Debt Instrument [Line Items]
Face amount											$ 2,100,000.0
FHLB Community Investment Cash Advance Program, Third Advance
Debt Instrument [Line Items]
Face amount										$ 3,000,000.0
FHLB Community Investment Cash Advance Program, Fourth Advance
Debt Instrument [Line Items]
Face amount			1,000,000.0
FHLB Community Investment Cash Advance Program, Final Advance
Debt Instrument [Line Items]
Face amount			$ 769,000
Note Agreement With Federal Reserve Bank
Debt Instrument [Line Items]
Loans pledged as collateral		$ 79,500,000										$ 79,500,000
Interest paid (less than)												1,000	$ 1,000
Discount Window
Debt Instrument [Line Items]
Loans pledged as collateral		$ 48,300,000										48,300,000
FHLB and Secured Debt
Debt Instrument [Line Items]
Interest expense on deposits, borrowings and junior subordinated debentures												$ 291,000	$ 273,000	$ 207,000