Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions		1 Months Ended	3 Months Ended		8 Months Ended
	Apr. 10, 2026	Apr. 30, 2026	Mar. 31, 2026	Mar. 31, 2025	Apr. 30, 2026	Apr. 28, 2026
Subsequent Event [Line Items]
Stock repurchased and retired (in shares)			2,477,093	106,630
Stock Repurchased and Retired During Period, Value			$ 87.7	$ 3.2
Deposits - branch sale			253.6
August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Value			$ 84.0
Subsequent Event
Subsequent Event [Line Items]
Deposits - branch sale	$ 244.2
Loans - branch sale	64.1
Sale and Leaseback Transaction, Gain (Loss), Net	$ 19.5
Subsequent Event | August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Value		$ 10.4			$ 212.0
Class A Common Stock
Subsequent Event [Line Items]
Stock repurchased and retired (in shares)			2,392,893
Class A Common Stock | Subsequent Event
Subsequent Event [Line Items]
Dividend amount per share (in dollars)						$ 0.47
Stock repurchased and retired (in shares)		303,000
August 28, 2025 FIBK Repurchase Program | Subsequent Event
Subsequent Event [Line Items]
Stock repurchased and retired (in shares)					6,400,000