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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Dec. 31, 2021
Share-based compensation arrangement by share-based payment award        
Target performance shares outstanding (in shares) 35,390      
Accrued Expense $ 72.2      
Assumed forfeitures (in shares) (531)      
Assumed forfeitures $ (1.1)      
Total target performance shares outstanding (in shares) 34,859 37,031 39,449 40,828
Total $ 71.1 $ 69.4 $ 67.5 $ 42.2
2024 – 2026        
Share-based compensation arrangement by share-based payment award        
Target performance shares outstanding (in shares) 11,405      
Accrued Expense $ 6.5      
2023 – 2025        
Share-based compensation arrangement by share-based payment award        
Target performance shares outstanding (in shares) 10,835      
Accrued Expense $ 14.4      
2022 – 2024        
Share-based compensation arrangement by share-based payment award        
Target performance shares outstanding (in shares) 13,150      
Accrued Expense $ 51.3