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Employee Share-Based Incentive Compensation Plans (Tables) | 12 Months Ended
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Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |
Schedule of performance share activity | The following table presents performance share activity for the years ended December 31, 2023, 2022 and 2021 for performance shares granted under the WTM Incentive Plan:
|  | Year Ended December 31,
|  | 2023 |  | 2022 |  | 2021
$ in Millions |  | Target Performance Shares Outstanding |  | Accrued Expense |  | Target Performance Shares Outstanding |  | Accrued Expense |  | Target Performance Shares Outstanding |  | Accrued Expense
Beginning of period |  | 39,449 |  |  | $ | 67.5 |  |  | 40,828 |  |  | $ | 42.2 |  |  | 42,458 |  |  | $ | 56.3 |
Shares paid or expired (1) |  | (13,350) |  |  | (40.8) |  |  | (14,625) |  |  | (26.4) |  |  | (14,336) |  |  | (35.2) |
New grants |  | 10,895 |  |  | — |  |  | 13,225 |  |  | — |  |  | 13,475 |  |  | — |
Forfeitures and cancellations (2) |  | 37 |  |  | (.1) |  |  | 21 |  |  | (.4) |  |  | (769) |  |  | .4 |
Expense recognized |  | — |  |  | 42.8 |  |  | — |  |  | 52.1 |  |  | — |  |  | 20.7 |
End of period |  | 37,031 |  |  | $ | 69.4 |  |  | 39,449 |  |  | $ | 67.5 |  |  | 40,828 |  |  | $ | 42.2 |
(1)WTM performance share payments in 2023 for the 2020-2022 performance cycle, which were paid in March 2023 at 200% of target. WTM performance share payments in 2022 for the 2019-2021 performance cycle, which were paid in March 2022 at 172% of target. WTM performance share payments in 2021 for the 2018-2020 performance cycle, which were paid in March 2021 at 200% of target. (2)Amounts include changes in assumed forfeitures, as required under GAAP.
Schedule of performance shares outstanding and accrued expense for performance shares awarded | The following table presents performance shares outstanding and accrued expense for performance shares awarded under the WTM Incentive Plan as of December 31, 2023 for each performance cycle:
|  | December 31, 2023
$ in Millions |  | Target Performance Shares Outstanding |  | Accrued Expense
Performance cycle: |  |  |  |
2023 – 2025 |  | 10,895 |  |  | $ | 6.0 |
2022 – 2024 |  | 13,225 |  |  | 25.8 |
2021 – 2023 |  | 13,475 |  |  | 38.7 |
Sub-total |  | 37,595 |  |  | 70.5 |
Assumed forfeitures |  | (564) |  |  | (1.1) |
Total |  | 37,031 |  |  | $ | 69.4 |
Schedule of unrecognized compensation costs associated with outstanding restricted share awards | The following table presents the unrecognized compensation cost associated with the outstanding restricted share awards under the WTM Incentive Plan for the years ended December 31, 2023, 2022 and 2021:
|  | Year Ended December 31,
|  | 2023 |  | 2022 |  | 2021
$ in Millions |  | Restricted Shares |  | Unamortized Issue Date Fair Value |  | Restricted Shares |  | Unamortized Issue Date Fair Value |  | Restricted Shares |  | Unamortized Issue Date Fair Value
Non-vested: |  |  |  |  |  |  |  |  |  |  |  |
Beginning of period |  | 38,350 |  |  | $ | 15.5 |  |  | 37,850 |  |  | $ | 15.9 |  |  | 43,105 |  |  | $ | 15.2 |
Issued |  | 10,895 |  |  | 16.0 |  |  | 13,225 |  |  | 13.8 |  |  | 13,475 |  |  | 16.1 |
Vested |  | (11,650) |  |  | — |  |  | (12,725) |  |  | — |  |  | (17,936) |  |  | — |
Forfeited |  | — |  |  | — |  |  | — |  |  | — |  |  | (794) |  |  | (.8) |
Expense recognized |  | — |  |  | (15.3) |  |  | — |  |  | (14.2) |  |  | — |  |  | (14.6) |
End of period |  | 37,595 |  |  | $ | 16.2 |  |  | 38,350 |  |  | $ | 15.5 |  |  | 37,850 |  |  | $ | 15.9 |

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