Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions		3 Months Ended		9 Months Ended
	Apr. 08, 2019 USD ($) office segment $ / shares shares	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 621.6		$ 621.6		$ 621.6
Acquisition related expenses		$ 0.0	$ 3.8	$ 0.0	$ 19.6
Core Deposits
Business Acquisition [Line Items]
Amortization of Intangible Assets	$ 3.0
Estimated useful lives of related deposits	10 years
Community 1st Bank
Business Acquisition [Line Items]
Number of banking offices acquired | office	3
Total consideration exchanged	$ 18.8
Goodwill	2.3
Provisional net assets acquired	16.5
Provisional assets recorded	129.1
Provisional loans recorded	78.8
Acquired debt impaired	0.7
Provisional liabilities recorded	112.6
Provisional deposits recorded	$ 110.1
Community 1st Bank | Class A Common Stock
Business Acquisition [Line Items]
Number of shares issued in connection with Cascade merger (in shares) | shares	463,134
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares	$ 40.64
Business acquisition conversion share rate (in shares) | shares	0.3784
Idaho Independent Bank
Business Acquisition [Line Items]
Number of banking offices acquired | office	11
Total consideration exchanged	$ 157.3
Goodwill	$ 73.0
Number of operating segments | segment	1
Core deposit intangible assets	$ 13.6
Idaho Independent Bank | Class A Common Stock
Business Acquisition [Line Items]
Number of shares issued in connection with Cascade merger (in shares) | shares	3,871,422
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares	$ 40.64
Business acquisition conversion share rate (in shares) | shares	0.50
Idaho Independent Bank | Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years