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Acquisitions - Business Combination (Narrative) (Details) $ / shares in Units, $ in Millions |  |  |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 11, 2018 USD ($) $ / shares | Aug. 16, 2018 USD ($) $ / shares shares | May 30, 2017 USD ($) $ / shares shares | Sep. 30, 2018 USD ($) | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) segment | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Goodwill |  |  |  | $ 546.4 | $ 445.0 | $ 546.4 | $ 445.0 | $ 444.7 | $ 212.8
Acquisition related expenses |  |  |  | 3.1 | 13.0 | 5.4 | 23.9 |  |
Net income |  |  |  | $ 41.4 | $ 27.3 | $ 119.8 | $ 72.3 |  |
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 |  |  |
Northwest Bank Acquisition, as Recorded by the Company [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Core deposit intangible assets |  | $ 15.7 |  |  |  |  |  |  |
Cash paid by First Interstate in connection with merger |  | $ 3.0 |  |  |  |  |  |  |
Inland Northwest Bank |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Number of banking offices acquired |  | 20 |  |  |  |  |  |  |
Cash Paid For Northwest Warrants |  | $ 3.0 |  |  |  |  |  |  |
Total consideration exchanged in connection with the merger |  | 176.3 |  |  |  |  |  |  |
Goodwill |  | $ 100.8 |  |  |  |  |  |  |
Number of operating segments | segment |  |  |  |  |  | 1 |  |  |
Inland Northwest Bank | Class A Common Stock |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business acquisition conversion share rate | shares |  | 0.516 |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger | shares |  | 3,840,000 |  |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares |  | $ 45.15 |  |  |  |  |  |  |
Inland Northwest Bank | Core Deposits |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Estimated useful lives of related deposits |  | 10 years |  |  |  |  |  |  |
Bank of the Cascades |  |  |  |  |  |  |  |  |
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 |  |  |  |  |  |
Cash paid by First Interstate in connection with merger |  |  | $ 155.0 |  |  |  |  |  |
Total consideration exchanged in connection with the merger |  |  | 541.0 |  |  |  |  |  |
Cash Paid Related To Cascade Options |  |  |  |  |  | $ 9.3 |  |  |
Cash Paid For Cascade Unvested Restricted Stock |  |  | 2.2 |  |  |  |  |  |
Expense related to restricted awards excluded from consideration |  |  | 2.4 |  |  |  |  |  |
Restricted stock expense included in consideration |  |  |  |  |  | $ 5.5 |  |  |
Goodwill |  |  | $ 232.8 |  |  |  |  |  |
Number of operating segments | segment |  |  |  |  |  | 1 |  |  |
Bank of the Cascades | Class A Common Stock |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Business acquisition conversion share rate | shares |  |  | 0.14864 |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares |  |  | $ 34.30 |  |  |  |  |  |
Shares Issued For Cascade Unvested Restricted Stock | shares |  |  | 168 |  |  |  |  |  |
Bank of the Cascades | Core Deposits |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Estimated useful lives of related deposits |  |  | 10 years |  |  |  |  |  |
Subsequent Event | Community 1st Bank [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Estimate value per share of Acquisition | $ / shares | $ 17.20 |  |  |  |  |  |  |  |
Subsequent Event | Inland Northwest Bank |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Estimate value per share of Acquisition | $ / shares | $ 22.73 |  |  |  |  |  |  |  |
Subsequent Event | Idaho Independent Bank [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Number of banking offices acquired | 11 |  |  |  |  |  |  |  |
Estimate of Payments to Acquire Businesses, Gross | $ 181.3 |  |  |  |  |  |  |  |
Subsequent Event | Community 1st Bank [Member] |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |
Number of banking offices acquired | 3 |  |  |  |  |  |  |  |
Estimate of Payments to Acquire Businesses, Gross | $ 21.5 |  |  |  |  |  |  |  |

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