Acquisitions - Business Combination (Narrative) (Details) $ / shares in Units, $ in Millions		3 Months Ended
	May 30, 2017 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) segment	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Business acquisition cash conversion amount (in dollars per share) | $ / shares	$ 1.91
Number of shares issued in connection with Cascade merger | shares	11,300,000
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares	$ 34.30
Cash Paid Related To Cascade Options		$ 9.3
Cash Paid For Cascade Unvested Restricted Stock	$ 2.2
Shares Issued For Cascade Unvested Restricted Stock | shares	168
Expense related to restricted awards excluded from consideration	$ 2.4
Restricted stock expense included in consideration		5.5
Goodwill		$ 445.6	$ 212.8	$ 444.7	$ 212.8
Number of operating segments | segment		1
Acquisition related expenses		$ 2.3	0.7
Bank of the Cascades
Business Acquisition [Line Items]
Number of banking offices acquired	46
Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits		10 years
Bank of the Cascades
Business Acquisition [Line Items]
Acquisition related expenses			$ 0.7
Bank of the Cascades | Class A Common Stock
Business Acquisition [Line Items]
Business acquisition conversion share rate | shares	0.14864
Bank of the Cascades | Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits	10 years
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Core deposit intangible assets	$ 48.0
Cash paid by First Interstate in connection with merger	155.0
Total consideration exchanged in connection with the merger	541.0
Goodwill	$ 232.8