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Acquisitions - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions |  | 3 Months Ended |
--- | --- | --- | ---
Feb. 01, 2022 USD ($) office $ / shares shares | Mar. 31, 2023 USD ($) | Mar. 31, 2022 USD ($) | Dec. 31, 2022 USD ($)
Business Acquisition [Line Items] |  |  |  |
Goodwill |  | $ 1,100.9 |  | $ 1,100.9
Acquisition related expenses |  | $ 0.0 | $ 65.2 |
Business Acquisition, Goodwill, Expected Tax Deductible Amount | $ 31.7 |  |  |
Core Deposits |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Estimated useful lives of related deposits |  | 10 years |  |
Great Western Bank |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Number of banking offices acquired | office | 174 |  |  |
Total consideration exchanged | $ 1,723.3 |  |  |
Goodwill | 479.3 |  |  |
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 | 46.9 |  |  |
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares | $ 36.76 |  |  |

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- Definition Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 350 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (a)(3) -URI https://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275 + Details
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- Definition This element represents acquisition-related costs incurred to effect a business combination which costs have been expensed during the period. Such costs include finder's fees; advisory, legal, accounting, valuation, and other professional or consulting fees; general administrative costs, including the costs of maintaining an internal acquisitions department; and may include costs of registering and issuing debt and equity securities. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -URI https://asc.fasb.org/extlink&oid=123586518&loc=d3e1043-128460 + Details
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