XML 75 R65.htm IDEA: XBRL DOCUMENT v3.25.3
Capital Stock (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2025
Sep. 30, 2024
Sep. 30, 2025
Sep. 30, 2024
Aug. 28, 2025
Dec. 31, 2024
Class of Stock [Line Items]            
Shares outstanding (in shares) 103,967,257   103,967,257     104,585,964
Stock repurchased and retired (in shares) 844,426 3,972 952,074 49,420    
Stock Repurchased and Retired During Period, Value $ 27.5 $ 0.1 $ 30.7 $ 1.2    
Common Stock, Shares, Issued 103,967,257   103,967,257     104,585,964
Capital Stock, Authorized 150,100,000   150,100,000      
Common Stock, Shares Authorized 150,000,000   150,000,000     150,000,000
Preferred Stock, Shares Authorized 100,000   100,000     100,000
Common shares issued (in shares) 3,706   39,058      
Share Repurchase Program, Authorized, Amount         $ 150.0  
Share Repurchase Program, Remaining Authorized, Amount $ 122.5   $ 122.5      
Shares repurchased, average cost per share (in dollars per share)     $ 32.58      
August 28, 2025 FIBK Repurchase Program            
Class of Stock [Line Items]            
Stock Repurchased and Retired During Period, Value     $ 27.5      
Director [Member]            
Class of Stock [Line Items]            
Common shares issued (in shares)     39,058      
Class A Common Stock            
Class of Stock [Line Items]            
Shares outstanding (in shares)           104,585,964
Stock repurchased and retired (in shares)     842,903      
Common Stock, Shares, Issued           104,585,964
Common Stock, Voting Rights, Votes Per Share     $ 1      
Class A Common Stock | Director [Member]            
Class of Stock [Line Items]            
Dividend, Share-based Payment Arrangement, Paid-in-Kind     $ 1.1