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v3.22.2.2

Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions |  | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | --- | ---
Feb. 01, 2022 USD ($) office $ / shares shares | Sep. 30, 2022 USD ($) | Sep. 30, 2021 USD ($) | Sep. 30, 2022 USD ($) | Sep. 30, 2021 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Business Acquisition [Line Items] |  |  |  |  |  |  |
Goodwill |  | $ 1,100.0 |  | $ 1,100.0 |  | $ 621.6 | $ 621.6
Estimated useful lives of related deposits |  |  |  | 12 years |  |  |
Acquisition related expenses |  | 4.0 | $ 6.6 | $ 115.0 | $ 6.6 |  |
Goodwill, Period Increase (Decrease) |  | 0.2 |  |  |  |  |
Core Deposits |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Estimated useful lives of related deposits | 10 years |  |  | 10 years |  |  |
Great Western Bank |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of banking offices acquired | office | 174 |  |  |  |  |  |
Total consideration exchanged | $ 1,723.3 |  |  |  |  |  |
Goodwill | 478.4 |  |  |  |  |  |
Core deposit intangibles | 50.1 |  |  |  |  |  |
Customer relationship intangible | 22.8 |  |  |  |  |  |
Great Western Bank | First Interstate Foundation |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Acquisition related costs | 21.5 |  |  |  |  |  |
Great Western Bank | Scott Family |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Acquisition related costs | $ 8.2 |  |  |  |  |  |
Great Western Bank | Class A Common Stock |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Number of shares issued in connection with Cascade merger (in shares) | shares | 46,879,601 |  |  |  |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 36.76 |  |  |  |  |  |
Great Western Bank | Customer Relationships |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Estimated useful lives of related deposits | 12 years |  |  |  |  |  |
Acquisition related costs as defined by ASC 805 |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Acquisition related expenses |  | $ 0.0 |  | $ 26.8 |  |  |

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