Acquisitions - Business Combination (Narrative) (Details)		1 Months Ended	3 Months Ended		9 Months Ended
	May 30, 2017 USD ($) segment $ / shares shares	Jun. 30, 2017 USD ($)	Sep. 30, 2017 USD ($) office shares	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) office shares	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Effective date of merger					May 30, 2017
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		$ 2,200,000
Shares issued for Cascade unvested restricted stock | shares			168,000		168,000
Expense related to restricted awards excluded from consideration	$ 2,400,000
Restricted stock expense included in consideration					$ 5,500,000
Goodwill			$ 445,028,000		445,028,000		$ 212,820,000
Number of operating segments | segment	1
Established through acquisitions or provisional adjustments			19,000	$ 2,486,000	47,968,000	$ 2,486,000
Acquisition related expenses			$ 13,016,000	$ 1,197,000	23,854,000	$ 1,197,000
Acquisition-related costs					$ 19,700,000
Bank of the Cascades
Business Acquisition [Line Items]
Number of banking offices | office			46		46
Class A Common Stock
Business Acquisition [Line Items]
Number of shares issued in connection with Cascade merger | shares	11,252,750
Bank of the Cascades
Business Acquisition [Line Items]
Date of agreement and plan for merger with Cascade Bancorp					Nov. 17, 2016
Valuation allowance for deferred tax assets			$ 0		$ 0
Acquisition related expenses			$ 13,016,000		$ 23,854,000
Revenue reported by Cascade		$ 12,900,000
Net income reported by Cascade		$ 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]
Established through acquisitions or provisional adjustments	$ 48,000,000
Estimated useful lives of related deposits	10 years				10 years
Cascade Bank Acquisition, as Recorded by the Company
Business Acquisition [Line Items]
Cash paid by First Interstate in connection with merger	$ 155,029,000
Total consideration exchanged in connection with the merger	540,998,000
Goodwill	$ 232,208,000