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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, 2023 | Dec. 31, 2022 | Dec. 31, 2021 | Dec. 31, 2020
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 37,595 |  |  |
Accrued Expense | $ 70.5 |  |  |
Assumed forfeitures (in shares) | (564) |  |  |
Assumed forfeitures | $ (1.1) |  |  |
Total target performance shares outstanding (in shares) | 37,031 | 39,449 | 40,828 | 42,458
Total | $ 69.4 | $ 67.5 | $ 42.2 | $ 56.3
2023 – 2025 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 10,895 |  |  |
Accrued Expense | $ 6.0 |  |  |
2022 – 2024 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 13,225 |  |  |
Accrued Expense | $ 25.8 |  |  |
2023 – 2025 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 13,475 |  |  |
Accrued Expense | $ 38.7 |  |  |

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