Acquisitions - Narrative (Details) $ / shares in Units, $ in Millions			3 Months Ended		6 Months Ended
	Apr. 08, 2019 USD ($) office segment $ / shares shares	Aug. 16, 2018 USD ($) office $ / shares shares	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 623.5		$ 623.5		$ 546.7	$ 444.7
Acquisition related expenses			$ 13.5	$ 0.0	$ 15.8	$ 2.3
Core Deposits
Business Acquisition [Line Items]
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	$ 2.5
Number of operating segments | segment	1
Provisional net assets acquired	$ 16.3
Provisional assets recorded	128.0
Provisional loans recorded	78.8
Acquired debt impaired	0.7
Provisional liabilities recorded	111.7
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
Community 1st Bank | Core Deposits
Business Acquisition [Line Items]
Core deposit intangible assets	$ 3.0
Estimated useful lives of related deposits	10 years
Idaho Independent Bank
Business Acquisition [Line Items]
Number of banking offices acquired | office	11
Total consideration exchanged	$ 157.3
Goodwill	$ 74.7
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]
Core deposit intangible assets	$ 13.6
Estimated useful lives of related deposits	10 years
Inland Northwest Bank
Business Acquisition [Line Items]
Number of banking offices acquired | office		20
Total consideration exchanged		$ 176.3
Goodwill		100.7
Number of operating segments | segment	1
Core deposit intangible assets		$ 15.7
Inland Northwest Bank | Class A Common Stock
Business Acquisition [Line Items]
Number of shares issued in connection with Cascade merger (in shares) | shares		3,840,000
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares		$ 45.15
Business acquisition conversion share rate (in shares) | shares		0.516
Inland Northwest Bank | Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits		10 years