Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		11 Months Ended
	Jul. 30, 2026	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025	Jul. 30, 2026	Jul. 22, 2026	Jan. 27, 2026	Aug. 28, 2025
Subsequent Event [Line Items]
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
August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Value		$ 68.7		$ 152.7
Share Repurchase Program, Authorized, Amount								$ 150.0	$ 150.0
Subsequent Event | August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Value	$ 9.3					$ 279.6
Share Repurchase Program, Authorized, Amount	$ 450.0					$ 450.0	$ 150.0
Class A Common Stock | August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock repurchased and retired (in shares)		1,929,769		4,322,662
Class A Common Stock | Subsequent Event
Subsequent Event [Line Items]
Dividend amount per share (in dollars)							$ 0.47
Class A Common Stock | Subsequent Event | August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock repurchased and retired (in shares)	243,000
August 28, 2025 FIBK Repurchase Program | Subsequent Event | August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock repurchased and retired (in shares)						8,200,000