Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2026	Mar. 31, 2025	Jan. 27, 2026	Dec. 31, 2025	Aug. 28, 2025
Class of Stock [Line Items]
Shares outstanding (in shares)	97,446,230			101,105,745
Stock repurchased and retired (in shares)	2,477,093	106,630
Stock Repurchased and Retired During Period, Value	$ 87.7	$ 3.2
Common Stock, Shares, Issued	97,446,230			101,105,745
Capital Stock, Authorized	150,100,000
Common Stock, Shares Authorized	150,000,000			150,000,000
Preferred Stock, Shares Authorized	100,000			100,000
Share Repurchase Program, Authorized, Amount	$ 300.0		$ 150.0		$ 150.0
Shares repurchased, average cost per share (in dollars per share)	$ 35.09
Share Repurchase Program, Remaining Authorized, Amount	$ 98.4
August 28, 2025 FIBK Repurchase Program
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 84.0
Class A Common Stock
Class of Stock [Line Items]
Shares outstanding (in shares)				101,105,745
Stock repurchased and retired (in shares)	2,392,893
Common Stock, Shares, Issued				101,105,745
Common Stock, Voting Rights, Votes Per Share	$ 1