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Employee Share-Based Incentive Compensation Plans - Performance Shares Granted Under the WTM Incentive Plan (Details) - Performance Share - W T M Incentive Plan - USD ($) $ in Millions | Jun. 30, 2023 | Mar. 31, 2023 | Dec. 31, 2022 | Jun. 30, 2022 | Mar. 31, 2022 | Dec. 31, 2021
--- | --- | --- | --- | --- | --- | ---
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 37,595 |  |  |  |  |
Assumed forfeitures (in shares) | (564) |  |  |  |  |
Total target performance shares outstanding (in shares) | 37,031 | 37,031 | 39,449 | 39,449 | 39,302 | 40,828
Accrued Expense | $ 45.4 |  |  |  |  |
Assumed forfeitures | (0.8) |  |  |  |  |
Total Accrued Expense | $ 44.6 | $ 36.4 | $ 67.5 | $ 41.2 | $ 21.9 | $ 42.2
2021 – 2023 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 13,475 |  |  |  |  |
Accrued Expense | $ 26.3 |  |  |  |  |
2022 – 2024 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 13,225 |  |  |  |  |
Accrued Expense | $ 17.3 |  |  |  |  |
2023 – 2025 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 10,895 |  |  |  |  |
Accrued Expense | $ 1.8 |  |  |  |  |

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