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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 | Mar. 31, 2022 | Dec. 31, 2021 | Mar. 31, 2021 | Dec. 31, 2020
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Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 39,900 |  |  |
Assumed forfeitures (in shares) | (598) |  |  |
Total target performance shares outstanding (in shares) | 39,302 | 40,828 | 40,784 | 42,458
Accrued Expense | $ 22.2 |  |  |
Assumed forfeitures | (0.3) |  |  |
Total Accrued Expense | $ 21.9 | $ 42.2 | $ 38.2 | $ 56.3
2020 – 2022 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 13,350 |  |  |
Accrued Expense | $ 19.0 |  |  |
2021 – 2023 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 13,475 |  |  |
Accrued Expense | $ 2.9 |  |  |
2022 – 2024 |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |
Target performance shares outstanding (in shares) | 13,075 |  |  |
Accrued Expense | $ 0.3 |  |  |

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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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- Definition Carrying value as of the balance sheet date for performance shares before forfeitures. + References No definition available. + Details
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- Definition This element represents the number of non-vested performance shares before forfeiture assumptions. + References No definition available. + Details
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