Acquisitions - Business Combination (Narrative) (Details) $ / shares in Units, $ in Millions				3 Months Ended		9 Months Ended
	Oct. 11, 2018 USD ($) $ / shares	Aug. 16, 2018 USD ($) $ / shares shares	May 30, 2017 USD ($) $ / shares shares	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) segment	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill				$ 546.4	$ 445.0	$ 546.4	$ 445.0	$ 444.7	$ 212.8
Acquisition related expenses				3.1	13.0	5.4	23.9
Net income				$ 41.4	$ 27.3	$ 119.8	$ 72.3
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
Northwest Bank Acquisition, as Recorded by the Company [Member]
Business Acquisition [Line Items]
Core deposit intangible assets		$ 15.7
Cash paid by First Interstate in connection with merger		$ 3.0
Inland Northwest Bank
Business Acquisition [Line Items]
Number of banking offices acquired		20
Cash Paid For Northwest Warrants		$ 3.0
Total consideration exchanged in connection with the merger		176.3
Goodwill		$ 100.8
Number of operating segments | segment						1
Inland Northwest Bank | Class A Common Stock
Business Acquisition [Line Items]
Business acquisition conversion share rate | shares		0.516
Number of shares issued in connection with Cascade merger | shares		3,840,000
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares		$ 45.15
Inland Northwest Bank | Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits		10 years
Bank of the Cascades
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
Cash paid by First Interstate in connection with merger			$ 155.0
Total consideration exchanged in connection with the merger			541.0
Cash Paid Related To Cascade Options						$ 9.3
Cash Paid For Cascade Unvested Restricted Stock			2.2
Expense related to restricted awards excluded from consideration			2.4
Restricted stock expense included in consideration						$ 5.5
Goodwill			$ 232.8
Number of operating segments | segment						1
Bank of the Cascades | Class A Common Stock
Business Acquisition [Line Items]
Business acquisition conversion share rate | shares			0.14864
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares			$ 34.30
Shares Issued For Cascade Unvested Restricted Stock | shares			168
Bank of the Cascades | Core Deposits
Business Acquisition [Line Items]
Estimated useful lives of related deposits			10 years
Subsequent Event | Community 1st Bank [Member]
Business Acquisition [Line Items]
Estimate value per share of Acquisition | $ / shares	$ 17.20
Subsequent Event | Inland Northwest Bank
Business Acquisition [Line Items]
Estimate value per share of Acquisition | $ / shares	$ 22.73
Subsequent Event | Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Number of banking offices acquired	11
Estimate of Payments to Acquire Businesses, Gross	$ 181.3
Subsequent Event | Community 1st Bank [Member]
Business Acquisition [Line Items]
Number of banking offices acquired	3
Estimate of Payments to Acquire Businesses, Gross	$ 21.5