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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2022 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 15,600 |  |
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2021 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  | 13,450 |
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2020 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  |  | 17,485
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2019 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  | 290 | 250
W T M Incentive and Phantom Plan | Restricted Share Cliff Vesting in January 2018 |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  | 365 | 250
White Mountains | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period | 41,510 | 53,755 | 70,620
Issued (in shares) | 15,600 | 14,105 | 17,985
Vested (in shares) | (13,715) | (25,381) | (28,846)
Assumed forfeitures and cancellations (in shares) | 0 | (969) | (6,004)
End of period | 43,395 | 41,510 | 53,755
Unamortized grant date fair value |  |  |
Beginning balance | $ 12.5 | $ 14.3 | $ 19.7
Issued | 14.5 | 11.4 | 16.3
Assumed forfeitures and cancellations | 0.0 | (0.2) | (3.5)
Expense recognized | (10.3) | (13.0) | (18.2)
Ending balance | $ 16.7 | $ 12.5 | $ 14.3
Sirius Group | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period |  | 2,195 |
End of period |  |  | 2,195

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