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Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Aug. 16, 2018 USD ($) | Dec. 31, 2007 USD ($) | Dec. 31, 2018 USD ($) trusts | Dec. 31, 2017 USD ($) | Jan. 31, 2008 USD ($) | Nov. 30, 2007 | Oct. 31, 2007 USD ($)
Subordinated Borrowing |  |  |  |  |  |  |
Number of company sponsored wholly-owned business trusts | trusts |  |  | 7 |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 86.9 | $ 82.5 |  |  |
Debt instrument, interest rate at period end |  |  | 5.55% |  |  |  |
Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Stated interest rate |  | 6.78% |  |  |  |  |
LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.75% |  |  |  |  |
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 10.3 | 10.3 |  |  | $ 10.3
Debt instrument, interest rate at period end |  |  | 5.05% |  |  |  |
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.25% |  |  |  |  |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 15.5 | 15.5 |  |  | $ 15.5
Debt instrument, interest rate at period end |  |  | 5.54% |  |  |  |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  | 7.50% |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.75% |  |  |  |  |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  | $ 20.6 | $ 20.6 | 20.6 |  |  |
Debt instrument, interest rate at period end |  |  | 5.19% |  |  |  |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Stated interest rate |  | 6.88% |  |  |  |  |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.40% |  |  |  |  |
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 15.5 | 15.5 |  |  |
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 10.3 | 10.3 | $ 10.3 |  |
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  |  | 5.50% |  |  |  |
Debt instrument, interest rate at period end |  |  | 2.70% |  |  |  |
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 10.3 | 10.3 | $ 15.5 |  |
Debt instrument, basis spread on variable rate |  |  | 2.75% |  |  |  |
Debt instrument, interest rate at period end |  |  | 1030000000.00% |  |  |  |
Northwest Bancorporation Capital Trust 1 [Member] | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  | $ 4.4 | $ 0.0 |  |  |
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Debt instrument, basis spread on variable rate | 1.70% |  |  |  |  |  |
Debt instrument, interest rate at period end | 4.50% |  |  |  |  |  |
Northwest Bancorporation Capital Trust I [Member] | Junior Subordinated Deferrable Interest Debentures Issued by NWBC I [Member] | Debt Instrument, Redemption, At Issuance through Year Five [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Stated interest rate | 5.95% |  |  |  |  |  |
Trust Preferred Securities Subject to Mandatory Redemption | Trusts |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption |  |  | $ 84.3 |  |  |  |
Business trust term (in years) |  |  | 30 years |  |  |  |
Trust Preferred Securities Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust 1 [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption | $ 5.0 |  |  |  |  |  |
Common Stock Subject to Mandatory Redemption | Trusts |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption |  |  | $ 2.6 |  |  |  |
Common Stock Subject to Mandatory Redemption | Northwest Bancorporation Capital Trust I [Member] |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption | $ 0.2 |  |  |  |  |  |
Subordinated Debentures Subject to Mandatory Redemption | Trusts |  |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |  |
Business trust term (in years) |  |  | 30 years |  |  |  |

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