Capital Stock (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Jan. 27, 2026	Dec. 31, 2025	Aug. 28, 2025
Class of Stock [Line Items]
Shares outstanding (in shares)	95,548,084		95,548,084			101,105,745
Stock repurchased and retired (in shares)	1,935,883	1,018	4,412,976	107,648
Stock Repurchased and Retired During Period, Value	$ 69.5	$ 0.0	$ 157.2	$ 3.2
Common Stock, Shares, Issued	95,548,084		95,548,084			101,105,745
Capital Stock, Authorized	150,100,000		150,100,000
Common Stock, Shares Authorized	150,000,000		150,000,000
Preferred Stock, Shares Authorized	100,000		100,000
August 28, 2025 FIBK Repurchase Program
Class of Stock [Line Items]
Stock Repurchased and Retired During Period, Value	$ 68.7		$ 152.7
Share Repurchase Program, Authorized, Amount					$ 150.0		$ 150.0
Shares repurchased, average cost per share (in dollars per share)	$ 35.61		$ 35.32
Share Repurchase Program, Remaining Authorized, Amount	$ 29.7		$ 29.7
Class A Common Stock
Class of Stock [Line Items]
Common Stock, Voting Rights, Votes Per Share			$ 1
Class A Common Stock | August 28, 2025 FIBK Repurchase Program
Class of Stock [Line Items]
Stock repurchased and retired (in shares)	1,929,769		4,322,662