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Acquisitions - Business Combination (Narrative) (Details) $ / shares in Units, $ in Millions |  |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | ---
Apr. 08, 2019 USD ($) $ / shares shares | Aug. 16, 2018 USD ($) $ / shares shares | Mar. 31, 2019 USD ($) | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) segment | Dec. 31, 2017 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |
Goodwill |  |  | $ 546.3 | $ 445.6 | $ 546.7 | $ 444.7
Acquisition related expenses |  |  | 2.3 | 2.3 | $ 2.3 |
Net income |  |  | $ 41.6 | $ 36.7 |  |
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 |  |  |  |
Total consideration exchanged in connection with the merger |  | $ 176.3 |  |  |  |
Goodwill |  | $ 100.7 |  |  |  |
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 |  |  |  |
Subsequent Event | Idaho Independent Bank [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Number of banking offices acquired | 11 |  |  |  |  |
Cash paid by First Interstate in connection with merger | $ 157.3 |  |  |  |  |
Subsequent Event | Idaho Independent Bank [Member] | Class A Common Stock |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition conversion share rate | shares | 0.5 |  |  |  |  |
Number of shares issued in connection with Cascade merger | shares | 3,871,000 |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 40.64 |  |  |  |  |
Subsequent Event | Community 1st Bank [Member] |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Number of banking offices acquired | 3 |  |  |  |  |
Cash paid by First Interstate in connection with merger | $ 18.8 |  |  |  |  |
Subsequent Event | Community 1st Bank [Member] | Class A Common Stock |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |
Business acquisition conversion share rate | shares | 0.3784 |  |  |  |  |
Number of shares issued in connection with Cascade merger | shares | 463,000 |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 40.64 |  |  |  |  |

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