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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 |  |  |

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