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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Apr. 08, 2019 USD ($) office segment $ / shares shares | Aug. 16, 2018 USD ($) office $ / shares shares | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Goodwill |  |  | $ 622.6 |  | $ 622.6 |  | $ 546.7 | $ 444.7
Acquisition related expenses |  |  | 3.8 | $ 3.1 | 19.6 | $ 5.4 |  |
Core Deposits |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Amortization of Intangible Assets | $ 3.0 |  | $ 65.0 |  | $ 65.0 |  | $ 56.9 |
Estimated useful lives of related deposits | 10 years |  |  |  | 10 years |  |  |
Community 1st Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of banking offices acquired | office | 3 |  |  |  |  |  |  |
Total consideration exchanged | $ 18.8 |  |  |  |  |  |  |
Goodwill | 2.2 |  |  |  |  |  |  |
Provisional net assets acquired | 16.6 |  |  |  |  |  |  |
Provisional assets recorded | 129.1 |  |  |  |  |  |  |
Provisional loans recorded | 78.8 |  |  |  |  |  |  |
Acquired debt impaired | 0.7 |  |  |  |  |  |  |
Provisional liabilities recorded | 112.5 |  |  |  |  |  |  |
Provisional deposits recorded | $ 110.1 |  |  |  |  |  |  |
Community 1st Bank | Class A Common Stock |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger (in shares) | shares | 463,134 |  |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 40.64 |  |  |  |  |  |  |
Business acquisition conversion share rate (in shares) | shares | 0.3784 |  |  |  |  |  |  |
Idaho Independent Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of banking offices acquired | office | 11 |  |  |  |  |  |  |
Total consideration exchanged | $ 157.3 |  |  |  |  |  |  |
Goodwill | $ 74.1 |  |  |  |  |  |  |
Number of operating segments | segment | 1 |  |  |  |  |  |  |
Core deposit intangible assets | $ 13.6 |  |  |  |  |  |  |
Idaho Independent Bank | Class A Common Stock |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger (in shares) | shares | 3,871,422 |  |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 40.64 |  |  |  |  |  |  |
Business acquisition conversion share rate (in shares) | shares | 0.50 |  |  |  |  |  |  |
Idaho Independent Bank | Core Deposits |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  |  |  |  |  |
Inland Northwest Bank |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of banking offices acquired | office |  | 20 |  |  |  |  |  |
Total consideration exchanged |  | $ 176.3 |  |  |  |  |  |
Goodwill |  | 100.7 |  |  |  |  |  |
Number of operating segments | segment | 1 |  |  |  |  |  |  |
Core deposit intangible assets |  | $ 15.7 |  |  |  |  |  |
Inland Northwest Bank | Class A Common Stock |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger (in shares) | shares |  | 3,840,000 |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares |  | $ 45.15 |  |  |  |  |  |
Business acquisition conversion share rate (in shares) | shares |  | 0.516 |  |  |  |  |  |
Inland Northwest Bank | Core Deposits |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |
Estimated useful lives of related deposits |  | 10 years |  |  |  |  |  |

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