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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 | Sep. 30, 2022 | Jun. 30, 2022 | Dec. 31, 2021 | Sep. 30, 2021 | Jun. 30, 2021 | Dec. 31, 2020
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Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 40,050 |  |  |  |  |
Assumed forfeitures (in shares) | (601) |  |  |  |  |
Total target performance shares outstanding (in shares) | 39,449 | 39,449 | 40,828 | 40,828 | 41,252 | 42,458
Accrued Expense | $ 53.8 |  |  |  |  |
Assumed forfeitures | (0.8) |  |  |  |  |
Total Accrued Expense | $ 53.0 | $ 41.2 | $ 42.2 | $ 38.8 | $ 45.7 | $ 56.3
2020 – 2022 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 13,350 |  |  |  |  |
Accrued Expense | $ 31.9 |  |  |  |  |
2021 – 2023 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 13,475 |  |  |  |  |
Accrued Expense | $ 14.7 |  |  |  |  |
2022 – 2024 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 13,225 |  |  |  |  |
Accrued Expense | $ 7.2 |  |  |  |  |

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- Definition Aggregate carrying value as of the balance sheet date of liabilities for incentive compensation arrangements. + References No definition available. + Details
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