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Subordinated Debentures Held by Subsidiary Trusts (Details) $ in Thousands | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jan. 31, 2008 USD ($) | Dec. 31, 2007 USD ($) | Nov. 30, 2007 USD ($) | Oct. 31, 2007 USD ($) | Dec. 31, 2017 USD ($) trusts | Dec. 31, 2016 USD ($)
Subordinated Borrowing |  |  |  |  |  |
Number of company sponsored wholly-owned business trusts | trusts |  |  |  |  | 6 |
Subordinated debentures held by subsidiary trusts |  |  |  |  | $ 82,477 | $ 82,477
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts |  |  |  | $ 10,300 | $ 10,300 | 10,300
Debt instrument, interest rate at period end |  |  |  |  | 3.94% |
First Interstate Statutory Trust II | Junior Subordinated Deferrable Interest Debentures Issued by FIST II | LIBOR |  |  |  |  |  |
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 |  |  | $ 15,500 |  | $ 15,500 | 15,500
Debt instrument, interest rate at period end |  |  |  |  | 4.34% |
First Interstate Statutory Trust I | Junior Subordinated Deferrable Interest Debentures Issued by FIST I | Debt Instrument, Redemption, At Issuance through Year Five |  |  |  |  |  |
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 |  | $ 20,600 |  |  | $ 20,600 | 20,600
Debt instrument, interest rate at period end |  |  |  |  | 3.99% |
First Interstate Statutory Trust III | Junior Subordinated Deferrable Interest Debentures Issued by FIST III | Debt Instrument, Redemption, At Issuance through Year Five |  |  |  |  |  |
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 |  | $ 15,500 |  |  | $ 15,500 | 15,500
Debt instrument, interest rate at period end |  |  |  |  | 4.39% |
First Interstate Statutory Trust IV | Junior Subordinated Deferrable Interest Debentures Issued by FIST IV | LIBOR |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Debt instrument, basis spread on variable rate |  | 2.70% |  |  |  |
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts | $ 10,300 |  |  |  | $ 10,300 | 10,300
Debt instrument, interest rate at period end |  |  |  |  | 4.44% |
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | Debt Instrument, Redemption, At Issuance through Year Five |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Stated interest rate | 6.78% |  |  |  |  |
First Interstate Statutory Trust V | Junior Subordinated Deferrable Interest Debentures Issued by FIST V | LIBOR | Debt Instrument, Redemption, After Period Five |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Debt instrument, basis spread on variable rate | 2.75% |  |  |  |  |
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Subordinated debentures held by subsidiary trusts | $ 10,300 |  |  |  | $ 10,300 | $ 10,300
Debt instrument, interest rate at period end |  |  |  |  | 4.44% |
First Interstate Statutory Trust VI | Junior Subordinated Deferrable Interest Debentures Issued by FIST VI | LIBOR |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Debt instrument, basis spread on variable rate | 2.75% |  |  |  |  |
Trust Preferred Securities Subject to Mandatory Redemption | Trusts |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption |  |  |  |  | $ 80,000 |
Business trust term (in years) |  |  |  |  | 30 years |
Common Stock Subject to Mandatory Redemption | Trusts |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Amount of financial instruments subject to mandatory redemption |  |  |  |  | $ 2,500 |
Subordinated Debentures Subject to Mandatory Redemption | Trusts |  |  |  |  |  |
Subordinated Borrowing |  |  |  |  |  |
Business trust term (in years) |  |  |  |  | 30 years |

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- Definition Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399 + Details
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- Definition Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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