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Reserves for Losses and Adjustment Expenses - Reconciliation of Beginning and Ending Consolidated Loss and Loss Adjustment Expenses (LAE) Reserves (Details) - USD ($)
$ in Millions
 9 Months Ended 12 Months Ended
Jan. 10, 2022
Sep. 30, 2024
Sep. 30, 2023
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
May 20, 2022
Dec. 31, 2020
Mar. 02, 2020
Insurance [Abstract]                  
Provision for losses and LAE, beginning balance   $ 7,810.6 $ 7,710.9 $ 7,710.9 $ 7,611.8 $ 7,165.3      
Less reinsurance recoverable       (4,577.8) (4,897.7) (3,298.1)   $ (3,195.2)  
Net loss and LAE expenses disposed (1)       0.0 1,840.1 0.0      
Net losses and LAE reserves, beginning balance   3,232.8 2,813.2 2,813.2 4,313.7 3,970.1      
Movement in net provision for losses and LAE for claims incurred:                  
Current year   1,239.2 1,137.1 1,492.2 1,651.9 1,648.2      
Prior years   39.7 10.5 60.8 28.1 45.1      
Total incurred       1,553.0 1,680.0 1,693.3      
Net Losses and LAE payments for claims incurred:                  
Current year   (109.9) (87.2) (161.1) (192.7) (729.1)      
Prior years   (655.0) (694.7) (1,011.9) (1,098.4) (580.7)      
Total paid   (764.9) (781.9) (1,173.0) (1,291.1) (1,309.8)      
Foreign exchange (gains)/losses   23.2 (4.8) 39.6 (49.3) (39.9)      
Net losses and LAE reserves, ending balance   3,770.0 3,174.1 3,232.8 2,813.2 4,313.7      
Plus reinsurance recoverable on unpaid losses at the end of the year       4,577.8 4,897.7 3,298.1      
Provision for losses and LAE at the end of the year   8,201.3 7,770.8 7,810.6 7,710.9 7,611.8      
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]                  
Reinsurance recoverable on unpaid losses       4,577.8 4,897.7        
Plus reinsurance recoverable on unpaid losses at the end of the year       4,577.8 4,897.7 3,298.1      
Reinsurance Contracts [Axis]: LPT                  
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]                  
Reinsurance recoverable total             $ (2,610.1)    
Reinsurance recoverables, premium paid and recognized             $ 1,840.1    
Reinsurance Contracts [Axis]: Retroactive Reinsurance                  
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]                  
Reinsurance recoverable on unpaid losses       1,627.4 2,132.0        
Insurance                  
Insurance [Abstract]                  
Provision for losses and LAE, beginning balance   1,859.7 1,452.5 1,452.5 2,165.3        
Net losses and LAE reserves, beginning balance   1,859.7 1,452.5 1,452.5 2,165.3        
Movement in net provision for losses and LAE for claims incurred:                  
Prior years   18.2 38.8            
Net Losses and LAE payments for claims incurred:                  
Net losses and LAE reserves, ending balance       1,859.7 1,452.5 2,165.3      
Provision for losses and LAE at the end of the year   2,115.0 1,769.3 $ 1,859.7 $ 1,452.5 $ 2,165.3      
Insurance | Reinsurance Contracts [Axis]: LPT, deferred gain                  
Movement in net provision for losses and LAE for claims incurred:                  
Prior years   17.6 23.7            
Insurance | Reinsurance Contracts [Axis]: LPT, offset post-LPT years                  
Movement in net provision for losses and LAE for claims incurred:                  
Prior years   $ 0.6 $ 15.1            
Enstar                  
Net Losses and LAE payments for claims incurred:                  
Plus reinsurance recoverable on unpaid losses at the end of the year                 $ 770.0
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]                  
Plus reinsurance recoverable on unpaid losses at the end of the year                 $ 770.0
Enstar | Reinsurance Contracts [Axis]: LPT net losses reinsured                  
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items]                  
Reinsurance, Excess Retention, Amount Reinsured, Per Event $ 3,120.0