Acquisitions - Business Combination (Narrative) (Details) $ / shares in Units, $ in Millions			3 Months Ended		12 Months Ended
	Apr. 08, 2019 USD ($) $ / shares shares	Aug. 16, 2018 USD ($) $ / shares shares	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) segment	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 546.3	$ 445.6	$ 546.7	$ 444.7
Acquisition related expenses			2.3	2.3	$ 2.3
Net income			$ 41.6	$ 36.7
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
Total consideration exchanged in connection with the merger		$ 176.3
Goodwill		$ 100.7
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
Subsequent Event | Idaho Independent Bank [Member]
Business Acquisition [Line Items]
Number of banking offices acquired	11
Cash paid by First Interstate in connection with merger	$ 157.3
Subsequent Event | Idaho Independent Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business acquisition conversion share rate | shares	0.5
Number of shares issued in connection with Cascade merger | shares	3,871,000
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares	$ 40.64
Subsequent Event | Community 1st Bank [Member]
Business Acquisition [Line Items]
Number of banking offices acquired	3
Cash paid by First Interstate in connection with merger	$ 18.8
Subsequent Event | Community 1st Bank [Member] | Class A Common Stock
Business Acquisition [Line Items]
Business acquisition conversion share rate | shares	0.3784
Number of shares issued in connection with Cascade merger | shares	463,000
Price per share of First Interstate Class A common stock (in dollars per share) | $ / shares	$ 40.64