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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 |  |  |  |  |

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