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COMMITMENTS, CONTINGENCIES AND OTHER ITEMS - Narrative (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Commitments and Contingencies [Line Items]    
Initial commitments $ 4,500.0 $ 3,600.0
Fulfilled commitments 2,500.0 2,000.0
Unfunded commitments $ 2,000.0 $ 1,600.0
Operating lease, weighted average lease term 5 years 10 months 24 days  
Operating Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] Other assets Other assets
Operating Lease, Liability, Statement of Financial Position [Extensible List] Other liabilities Other liabilities
Operating lease, right-of-use asset $ 83.8 $ 72.9
Operating lease, liability 114.3 103.9
Operating lease, expense $ 14.8 $ 12.8
Lessee, finance lease, term of contract 20 years  
Lessee, finance lease, renewal term 30 years  
Finance Lease, Right-of-Use Asset, Statement of Financial Position [Extensible List] Other assets Other assets
Finance Lease, Liability, Statement of Financial Position [Extensible List] Other liabilities Other liabilities
Finance lease, right-of-use asset $ 16.1 $ 16.6
Finance lease, liability 21.0 21.5
Letter of Credit    
Commitments and Contingencies [Line Items]    
Letters of credit outstanding 619.6  
Letter of Credit | Top Layer Reinsurance Ltd.    
Commitments and Contingencies [Line Items]    
Line of credit facility, maximum borrowing capacity 37.5  
Letter of Credit | Capital Support Agreement | Top Layer Reinsurance Ltd.    
Commitments and Contingencies [Line Items]    
Mandatory capital contribution in the event of capital and surplus reduction below a specified level 50.0  
Interest Expense    
Commitments and Contingencies [Line Items]    
Finance lease, interest expense 2.1 2.2
Other income (loss)    
Commitments and Contingencies [Line Items]    
Finance lease, right-of-use asset, amortization $ 0.5 $ 0.5