Capital Stock and Dividend Restrictions - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023	Aug. 28, 2025	May 23, 2024	May 25, 2023
Class of Stock
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares, outstanding	101,105,745	104,585,964
Payments for Repurchase of Common Stock	$ 121.9	$ 1.2	$ 34.0
Stock Repurchased and Retired During Period, Shares	3,764,290	49,568	1,056,191
Preferred Stock, Shares Authorized	100,000	100,000
Capital Stock, Authorized	150,100,000
Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Authorized					2,000,000	2,000,000
Share Repurchase Program, Authorized, Amount				$ 150.0
Stock Repurchased and Retired During Period, Value	$ 121.9	$ 1.2	$ 34.0
Shares Acquired, Average Cost Per Share	$ 32.18
Share Repurchase Program, Remaining Authorized, Amount	$ 32.4
August 28, 2025 FIBK Repurchase Program
Class of Stock
Stock Repurchased and Retired During Period, Value	$ 117.6
Class A Common Stock
Class of Stock
Common Stock, Voting Rights, Votes Per Share	$ 1
Common stock, shares, outstanding		104,585,964
Stock Repurchased and Retired During Period, Shares	3,653,914
Director [Member] | Class A Common Stock
Class of Stock
Shares issued to directors during period	39,058
Stock compensation expense	$ 1.1
Board of Directors
Class of Stock
Aggregate value of shares issued to directors		$ 1.2	$ 1.2
Board of Directors | Class A Common Stock
Class of Stock
Shares issued to directors during period		43,514	54,414