Acquisitions - Business Combination (Narrative) (Details)			1 Months Ended	3 Months Ended		6 Months Ended
	Apr. 25, 2018 USD ($) $ / shares	May 30, 2017 USD ($) $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) segment	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Pro Forma Acquisition Expenses, Net of Tax				$ 10,700,000		$ 11,200,000
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						9,300,000
Cash Paid For Cascade Unvested Restricted Stock		$ 2,200,000
Shares Issued For Cascade Unvested Restricted Stock | shares		168
Expense related to restricted awards excluded from consideration		$ 2,400,000
Restricted stock expense included in consideration						5,500,000
Goodwill			$ 444,300,000	445,600,000	$ 444,300,000	$ 445,600,000	$ 444,300,000	$ 444,700,000	$ 212,800,000
Number of operating segments | segment						1
Acquisition related expenses				0	10,100,000	$ 2,300,000	10,800,000
Net income				$ 41,700,000	$ 21,800,000	$ 78,400,000	$ 45,000,000
Inland Northwest Bank [Member]
Business Acquisition [Line Items]
Number of banking offices acquired	20
Estimate of Payments to Acquire Businesses, Gross	$ 159.7
Estimate value per share of Acquisition | $ / shares	$ 20.69
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]
Revenues			12,900,000
Net income			$ 3,000,000
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		$ 48,000,000
Cash paid by First Interstate in connection with merger		155,000,000
Total consideration exchanged in connection with the merger		541,000,000
Goodwill		$ 232,800,000