Acquisitions - Narrative (Details) $ / shares in Units, shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
	Feb. 01, 2022 USD ($) office $ / shares shares	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 1,100.9		$ 1,100.9		$ 1,100.9
Acquisition related expenses		$ 0.0	$ 45.8	$ 0.0	$ 111.0
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