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Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions |  | 12 Months Ended
--- | --- | ---
Feb. 01, 2022 USD ($) office $ / shares shares | Dec. 31, 2023 USD ($) segment | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Business Acquisition [Line Items] |  |  |  |
Finite-Lived Intangible Assets, Net |  | $ 81.4 | $ 97.0 |
Goodwill |  | $ 1,100.9 | 1,100.9 | $ 621.6
Number of operating segments | segment |  | 1 |  |
Finite-lived intangible assets acquired |  |  | 72.9 |
Estimated useful lives of related deposits |  | 12 years |  |
Acquisition related expenses |  | $ 0.0 | 118.9 | 11.6
Core Deposits Intangibles |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Finite-Lived Intangible Assets, Net |  | $ 62.2 | 75.9 |
Finite-lived intangible assets acquired |  |  | 50.1 |
Estimated useful lives of related deposits | 10 years | 10 years |  |
Great Western Bank |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Core deposit intangible assets | $ 50.1 |  |  |
Number of banking offices | office | 174 |  |  |
Total consideration paid | $ 1,723.3 |  |  |
Goodwill | 479.3 |  |  |
Great Western Bank | First Interstate Foundation |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Costs | 21.5 |  |  |
Great Western Bank | Scott Family |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business Combination, Separately Recognized Transactions, Additional Disclosures, Acquisition Costs | $ 8.2 |  |  |
Great Western Bank | Class A Common Stock |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Common stock issued (in shares) | shares | 46,879,601 |  |  |
Business Acquisition, Share Price | $ / 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 |  |  | $ 26.8 | $ 9.0

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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 -Topic 350 -SubTopic 30 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org//1943274/2147482665/350-30-50-1 + 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 -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 23 -Publisher FASB -URI https://asc.fasb.org//1943274/2147479405/805-10-25-23 + Details
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