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Debt and Financing Arrangements - Letter of Credit and Revolving Credit Facilities (Details) - USD ($)
Dec. 31, 2025
Dec. 31, 2024
Line of Credit Facility [Line Items]    
Advances from Federal Home Loan Banks $ 0 $ 0
Credit Facility    
Line of Credit Facility [Line Items]    
Optional increased capacity 1,500,000,000  
Credit Facility | Asset Pledged as Collateral | Letter of Credit    
Line of Credit Facility [Line Items]    
Investments securing LOCs 498,000,000  
Secured letter of credit facility    
Line of Credit Facility [Line Items]    
Maximum borrowing capacity 425,000,000  
Outstanding borrowings 224,000,000  
Remaining capacity 201,000,000  
Unsecured revolving loan and letter of credit facility    
Line of Credit Facility [Line Items]    
Maximum borrowing capacity 500,000,000  
Outstanding borrowings 0 $ 0
Remaining capacity 500,000,000  
Wholly-owned subsidiaries | Arch Re Bermuda | Unsecured Letter of Credit Facility    
Line of Credit Facility [Line Items]    
Maximum borrowing capacity 175,000,000  
Remaining capacity 46,000,000  
Letters of credit outstanding, amount 129,000,000  
Wholly-owned subsidiaries | Arch Re Bermuda | Lloyds Facility    
Line of Credit Facility [Line Items]    
Maximum borrowing capacity 700,000,000  
Letters of credit outstanding, amount 700,000,000  
Wholly-owned subsidiaries | Arch Re Bermuda | LOC Facilities    
Line of Credit Facility [Line Items]    
Letters of credit outstanding, amount $ 52,000,000