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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - USD ($) $ in Millions | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 | Dec. 31, 2022
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 34,525 |  |  |
Assumed forfeitures (in shares) | (518) |  |  |
Total target performance shares outstanding (in shares) | 34,007 | 37,031 | 37,031 | 39,449
Accrued Expense | $ 45.0 |  |  |
Assumed forfeitures | (0.8) |  |  |
Total | $ 44.2 | $ 69.4 | $ 36.4 | $ 67.5
2022 – 2024 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 13,225 |  |  |
Accrued Expense | $ 35.0 |  |  |
2023 – 2025 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 10,895 |  |  |
Accrued Expense | $ 9.4 |  |  |
2024 – 2026 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 10,405 |  |  |
Accrued Expense | $ 0.6 |  |  |

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