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Employee Share-Based Incentive Compensation Plans (WTM Restricted Shares) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Restricted Share Cliff Vesting in January 2017 [Member] |  |  |
Target Performance Share activity |  |  |
Issued (in shares) |  |  | 23,440
W T M Incentive and Phantom Plan [Member] | Restricted Share Cliff Vesting in January 2019 [Member] |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 24,615 |  |
W T M Incentive and Phantom Plan [Member] | Restricted Share Cliff Vesting in January 2018 [Member] |  |  |
Target Performance Share activity |  |  |
Issued (in shares) | 750 | 23,640 |
White Mountains | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period | 70,675 | 83,314 | 94,130
Issued (in shares) | 25,365 | 23,640 | 23,440
Vested (in shares) | (24,620) | (36,279) | (33,205)
Assumed forfeitures and cancellations (in shares) | (800) | 0 | (1,051)
End of period (3) | 70,620 | 70,675 | 83,314
Unamortized grant date fair value |  |  |
Beginning balance | $ 15.7 | $ 14.3 | $ 17.0
Issued | 20.2 | 15.7 | 13.1
Assumed forfeitures and cancellations | (0.3) | 0.0 | (0.5)
Expense recognized | (15.9) | (14.3) | (15.3)
Ending balance | $ 19.7 | $ 15.7 | $ 14.3
White Mountains | W T M Incentive Plan | Performance Shares |  |  |
Target Performance Share activity |  |  |
Beginning of period | 93,654 | 111,257 | 108,605
Assumed forfeitures and cancellations (in shares) | 378 | (3,839) | (3,208)
End of period (3) | 80,353 | 93,654 | 111,257
Unamortized grant date fair value |  |  |
Assumed forfeitures and cancellations | $ 0.5 | $ (0.3) | $ (0.1)
Expense recognized | (25.2) | $ (44.4) | $ (14.1)
Ending balance | $ 23.1 |  |
Sirius Group | W T M Incentive Plan | Restricted Stock |  |  |
Target Performance Share activity |  |  |
Beginning of period | 9,205 | 12,477 |
End of period (3) | 5,235 | 9,205 | 12,477
Sirius Group | W T M Incentive Plan | Performance Shares |  |  |
Target Performance Share activity |  |  |
Beginning of period | 10,826 | 12,607 |
End of period (3) | 7,315 | 10,826 | 12,607
Range of Performance Cycle from 2011 to 2013 [Member] | Performance Shares |  |  |
Target Performance Share activity |  |  |
Vested (in shares) |  |  | (3,570)
Range of Performance Cycle from 2011 to 2013 [Member] | W T M Incentive and Phantom Plan [Member] | Performance Shares |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage |  |  | 131.50%
Range of Performance Cycle from 2011 to 2013 [Member] | Long Term Incentive Plan |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Percentage of payout for minimum growth target percentage |  |  | 37.10%
Range Of Performance Cycle From 2014-2016 [Member] | W T M Incentive Plan | Performance Shares |  |  |
Share-based compensation arrangement by share-based payment award |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Payout for Maximum Growth Target Percentage | 200.00% |  |

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