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Acquisitions - Business Combination (Narrative) (Details) $ / shares in Units, $ in Millions |  | 3 Months Ended |  |
--- | --- | --- | --- | ---
May 30, 2017 USD ($) $ / shares shares | Mar. 31, 2018 USD ($) segment | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition cash conversion amount (in dollars per share) | $ / shares | $ 1.91 |  |  |  |
Number of shares issued in connection with Cascade merger | shares | 11,300,000 |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 34.30 |  |  |  |
Cash Paid Related To Cascade Options |  | $ 9.3 |  |  |
Cash Paid For Cascade Unvested Restricted Stock | $ 2.2 |  |  |  |
Shares Issued For Cascade Unvested Restricted Stock | shares | 168 |  |  |  |
Expense related to restricted awards excluded from consideration | $ 2.4 |  |  |  |
Restricted stock expense included in consideration |  | 5.5 |  |  |
Goodwill |  | $ 445.6 | $ 212.8 | $ 444.7 | $ 212.8
Number of operating segments | segment |  | 1 |  |  |
Acquisition related expenses |  | $ 2.3 | 0.7 |  |
Bank of the Cascades |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Number of banking offices acquired | 46 |  |  |  |
Core Deposits |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of related deposits |  | 10 years |  |  |
Bank of the Cascades |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Acquisition related expenses |  |  | $ 0.7 |  |
Bank of the Cascades | Class A Common Stock |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Business acquisition conversion share rate | shares | 0.14864 |  |  |  |
Bank of the Cascades | Core Deposits |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |  |
Cascade Bank Acquisition, as Recorded by the Company |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |
Core deposit intangible assets | $ 48.0 |  |  |  |
Cash paid by First Interstate in connection with merger | 155.0 |  |  |  |
Total consideration exchanged in connection with the merger | 541.0 |  |  |  |
Goodwill | $ 232.8 |  |  |  |

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