Amount authorized for purchase of share under share repurchase plan. Includes, but is not limited to, repurchase of stock and unit of ownership.

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The per share amount of a dividend declared, but not paid, as of the financial reporting date.

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Number of shares that have been repurchased and retired during the period.

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Equity impact of the value of stock that has been repurchased and retired during the period. The excess of the purchase price over par value can be charged against retained earnings (once the excess is fully allocated to additional paid in capital).

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Subsequent Events Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	2 Months Ended	6 Months Ended	12 Months Ended
	Feb. 20, 2026	Feb. 20, 2026	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Jan. 27, 2026	Aug. 28, 2025
Subsequent Event [Line Items]
Stated interest rate				4.77%
Stock Repurchased and Retired During Period, Shares			3,764,290	49,568	1,056,191
Share Repurchase Program, Authorized, Amount							$ 150.0
Stock Repurchased and Retired During Period, Value			$ 121.9	$ 1.2	$ 34.0
August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Value			$ 117.6
Subsequent Event
Subsequent Event [Line Items]
Share Repurchase Program, Authorized, Amount	$ 300.0	$ 300.0				$ 150.0
Subsequent Event \| August 28, 2025 FIBK Repurchase Program
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Value	$ 23.0	$ 140.6
Class A Common Stock
Subsequent Event [Line Items]
Stock Repurchased and Retired During Period, Shares			3,653,914
Class A Common Stock \| Subsequent Event
Subsequent Event [Line Items]
Dividends (in dollars per share)						$ 0.47
Stock Repurchased and Retired During Period, Shares	600,000	4,300,000