v3.26.1
ACQUISITION AND DIVESTITURE ACTIVITY - Additional Information (Details) - Bremer
 3 Months Ended 6 Months Ended
May 23, 2025
USD ($)
shares
May 01, 2025
USD ($)
$ / shares
Nov. 25, 2024
USD ($)
shares
Jun. 30, 2026
USD ($)
Jun. 30, 2025
USD ($)
Jun. 30, 2026
USD ($)
Jun. 30, 2025
USD ($)
Business Combination [Line Items]              
Cash per share above exchange (in dollars per share) | $ / shares   $ 26.22          
Shares exchange ratio   4.182          
Expected tax deductible amount   $ 0          
Other intangible assets   440,099,000          
Purchase price of loans at acquisition   1,876,226,000          
Par value of acquired loans at acquisition   2,055,598,000          
Allowance for credit losses at acquisition   103,546,000          
Transaction costs, expensed       $ 12,000,000.0 $ 40,200,000 $ 18,600,000 $ 40,900,000
Core deposit              
Business Combination [Line Items]              
Other intangible assets   $ 397,100,000          
Estimated useful lives   10 years          
Customer relationship              
Business Combination [Line Items]              
Other intangible assets   $ 43,000,000.0          
Estimated useful lives   12 years          
Citibank, N.A              
Business Combination [Line Items]              
Forward contract indexed to issuer's equity (in shares) | shares     19,047,619        
Forward contract indexed to equity, settlement, cash     $ 400,000,000.0        
Citigroup Global Markets Inc.              
Business Combination [Line Items]              
Forward contract indexed to issuer's equity (in shares) | shares 21,904,762            
Forward contract indexed to equity, settlement, cash $ 443,200,000            
Forward contrac indexed to issuers equity, period granted to purchase additional shares (in days)     30 days        
Additional number of shares to purchase (in shares) | shares     2,857,143        
Additional number of shares (in shares) | shares     2,857,143