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Employee Share-Based Incentive Compensation Plans (Tables) | 12 Months Ended
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Dec. 31, 2025
Share-Based Payment Arrangement [Abstract] |
Schedule of performance share activity | The following table presents performance share activity for the years ended December 31, 2025, 2024 and 2023 for performance shares granted under the WTM Incentive Plan:
|  | Year Ended December 31,
|  | 2025 |  | 2024 |  | 2023
$ in Millions |  | Target Performance Shares Outstanding |  | Accrued Expense |  | Target Performance Shares Outstanding |  | Accrued Expense |  | Target Performance Shares Outstanding |  | Accrued Expense
Beginning of period |  | 34,859 |  |  | $ | 71.1 |  |  | 37,031 |  |  | $ | 69.4 |  |  | 39,449 |  |  | $ | 67.5 |
Shares paid or expired (1) |  | (13,150) |  |  | (48.7) |  |  | (13,475) |  |  | (44.9) |  |  | (13,350) |  |  | (40.8) |
New grants |  | 10,670 |  |  | — |  |  | 11,405 |  |  | — |  |  | 10,895 |  |  | — |
Forfeitures and cancellations (2) |  | 37 |  |  | — |  |  | (102) |  |  | — |  |  | 37 |  |  | (.1) |
Expense recognized |  | — |  |  | 50.8 |  |  | — |  |  | 46.6 |  |  | — |  |  | 42.8 |
End of period |  | 32,416 |  |  | $ | 73.2 |  |  | 34,859 |  |  | $ | 71.1 |  |  | 37,031 |  |  | $ | 69.4 |
(1)WTM performance share payments for the 2022-2024 performance cycle were made in March 2025 at 200% of target. WTM performance share payments for the 2021-2023 performance cycle were made in March 2024 at 188% of target. WTM performance share payments for the 2020-2022 performance cycle were made in March 2023 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, 2025 for each performance cycle:
|  | December 31, 2025
$ in Millions |  | Target Performance Shares Outstanding |  | Accrued Expense
Performance cycle: |  |  |  |
2023 – 2025 |  | 10,835 |  |  | $ | 41.3 |
2024 – 2026 |  | 11,405 |  |  | 23.0 |
2025 – 2027 |  | 10,670 |  |  | 10.0 |
Sub-total |  | 32,910 |  |  | 74.3 |
Assumed forfeitures |  | (494) |  |  | (1.1) |
Total |  | 32,416 |  |  | $ | 73.2 |
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, 2025, 2024 and 2023:
|  | Year Ended December 31,
|  | 2025 |  | 2024 |  | 2023
$ 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 |  | 35,390 |  |  | $ | 19.9 |  |  | 37,595 |  |  | $ | 16.2 |  |  | 38,350 |  |  | $ | 15.5 |
Vested |  | (13,150) |  |  | — |  |  | (13,475) |  |  | — |  |  | (11,650) |  |  | — |
Issued |  | 10,670 |  |  | 19.6 |  |  | 11,405 |  |  | 20.0 |  |  | 10,895 |  |  | 16.0 |
Forfeited |  | — |  |  | — |  |  | (135) |  |  | — |  |  | — |  |  | — |
Expense recognized |  | — |  |  | (18.5) |  |  | — |  |  | (16.3) |  |  | — |  |  | (15.3) |
End of period |  | 32,910 |  |  | $ | 21.0 |  |  | 35,390 |  |  | $ | 19.9 |  |  | 37,595 |  |  | $ | 16.2 |

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