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