Quarterly report [Sections 13 or 15(d)]

BORROWINGS (Narrative) (Details)

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BORROWINGS (Narrative) (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2026
Sep. 30, 2026
Sep. 30, 2025
Dec. 31, 2025
Subordinated Borrowing [Line Items]        
Federal funds lines with correspondent banks $ 1,388,000 $ 1,388,000   $ 1,410,000
Maximum collateral dependent line of credit with the FHLB 11,400,000 11,400,000   11,100,000
Alternative line of credit with correspondent bank 25,000 25,000   25,000
Federal Reserve Discount Window 1,212,083 1,212,083   2,573,492
Federal Reserve Discount Window, outstanding 0 0   0
FHLB secured line of credit 4,898,679 4,898,679   5,277,231
Long-term debt, early repayments   168,000 $ 200,000  
Subordinated Debt 690,000 $ 690,000   $ 608,000
Basis spread on variable rate   0.262%   0.262%
Pre-tax loss on debt extinguishment   $ (2,654) $ 0  
FHLB Secured Line Of Credit        
Subordinated Borrowing [Line Items]        
Maximum collateral dependent line of credit with the FHLB 5,500,000 5,500,000   $ 5,900,000
2031 Subordinate Notes        
Subordinated Borrowing [Line Items]        
Subordinated Debt 250,000 $ 250,000   $ 250,000
Basis spread on variable rate   0.00%   0.00%
2036 Subordinated Notes        
Subordinated Borrowing [Line Items]        
Subordinated Debt $ 250,000 $ 250,000    
Subordinated debt rate 6.25%      
Basis spread on variable rate 2.13% 0.00%    
Net proceeds from issuance $ 246,800      
Sandy Spring | 2029 Subordinated Notes        
Subordinated Borrowing [Line Items]        
Subordinated Debt $ 168,000 $ 168,000   $ 168,000
Subordinated debt rate 4.25%      
Basis spread on variable rate       2.62%
Pre-tax loss on debt extinguishment $ 2,700