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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($)
$ in Millions
 9 Months Ended
Sep. 30, 2024
Jun. 30, 2024
Dec. 31, 2023
Sep. 30, 2023
Jun. 30, 2023
Dec. 31, 2022
Share-based compensation arrangement by share-based payment award            
Target performance shares outstanding (in shares) 35,390          
Assumed forfeitures (in shares) (531)          
Total target performance shares outstanding (in shares) 34,859 34,992 37,031 37,031 37,031 39,449
Accrued Expense $ 56.5          
Assumed forfeitures (0.8)          
Total $ 55.7 $ 52.0 $ 69.4 $ 55.1 $ 44.6 $ 67.5
2022 – 2024            
Share-based compensation arrangement by share-based payment award            
Target performance shares outstanding (in shares) 13,150          
Accrued Expense $ 40.7          
2023 – 2025            
Share-based compensation arrangement by share-based payment award            
Target performance shares outstanding (in shares) 10,835          
Accrued Expense $ 11.8          
2024 – 2026            
Share-based compensation arrangement by share-based payment award            
Target performance shares outstanding (in shares) 11,405          
Accrued Expense $ 4.0