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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, 2020 | Jun. 30, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Dec. 31, 2018
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Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 43,105 |  |  |  |  |
Assumed forfeitures (in shares) | (647) |  |  |  |  |
Total target performance shares outstanding (in shares) | 42,458 | 42,458 | 42,473 | 42,473 | 42,473 | 40,616
Accrued Expense | $ 29.6 |  |  |  |  |
Assumed forfeitures | (0.5) |  |  |  |  |
Total Accrued Expense | $ 29.1 | $ 15.8 | $ 43.7 | $ 36.4 | $ 30.8 | $ 31.7
Performance Cycle 2018 to 2020 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 13,450 |  |  |  |  |
Accrued Expense | $ 15.4 |  |  |  |  |
Performance Cycle 2019 to 2021 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 15,600 |  |  |  |  |
Accrued Expense | $ 10.6 |  |  |  |  |
Performance Cycle 2020 to 2022 |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Target performance shares outstanding (in shares) | 14,055 |  |  |  |  |
Accrued Expense | $ 3.6 |  |  |  |  |

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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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