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Borrowings - Narrative (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 22, 2013 | Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Jun. 30, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio of line of credit to eligible assets |  | 45.00% |  |  |  |  |  |  |  |  | 45.00% |  |
Line of credit |  | $ 139,300,000.0 |  |  |  |  |  |  |  |  | $ 139,300,000.0 |  |
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 |  | $ 667,800,000 |  |  |  |  |  |  |  |  | $ 667,800,000 |  |
Long-term FHLB advances |  | 7,200,000 |  |  |  | $ 7,400,000 |  |  |  |  | 7,200,000 | $ 7,400,000 |
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) |  |  |  |  |  |  |  |  |  |  | $ 2,971,000 | 2,470,000 | $ 2,553,000
Alaska limit on borrowing, percent of total assets |  |  |  |  |  |  |  |  |  |  | 15.00% |  |
Alaska limit on borrowing, total assets |  | 222,600,000 |  |  |  | 225,800,000 |  |  |  |  | $ 222,600,000 | 225,800,000 |
Securities sold under agreements to repurchase |  | $ 34,278,000 |  |  |  | $ 27,746,000 |  |  |  |  | $ 34,278,000 | $ 27,746,000 |
Percentage paid on securities sold under agreements to repurchase |  | 0.17% |  |  |  | 0.14% |  |  |  |  | 0.17% | 0.14% |
Average balance outstanding |  | $ 29,900,000 |  |  |  | $ 29,000,000 |  |  |  |  | $ 29,900,000 | $ 29,000,000 |
Maximum amount outstanding at any month end |  |  |  |  |  |  |  |  |  |  | 36,500,000 | 32,600,000 |
Interest expense on deposits, borrowings and junior subordinated debentures |  | $ 1,070,000 | $ 761,000 | $ 606,000 | $ 532,000 | $ 548,000 | $ 602,000 | $ 648,000 | $ 632,000 |  | $ 2,969,000 | $ 2,430,000 | $ 2,561,000
Weighted average interest rate |  | 3.39% |  |  |  | 3.23% |  |  |  |  | 3.39% | 3.23% | 2.82%
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 |  | $ 78,600,000 |  |  |  |  |  |  |  |  | $ 78,600,000 |  |
Interest paid (less than) |  |  |  |  |  |  |  |  |  |  | 1,000 | $ 1,000 |
Discount Window |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Loans pledged as collateral |  | $ 48,900,000 |  |  |  |  |  |  |  |  | 48,900,000 |  |
FHLB and Secured Debt |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest expense on deposits, borrowings and junior subordinated debentures |  |  |  |  |  |  |  |  |  |  | $ 273,000 | $ 207,000 | $ 157,000

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