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Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Mar. 01, 2012 Installment shares | May. 31, 2011 shares | Dec. 31, 2015 USD ($) shares | Dec. 31, 2014 USD ($) shares | Dec. 31, 2013 USD ($) shares | May. 25, 2011 Installment
Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) |  |  | 226,778 |  |  |
End of period (4) |  |  | 200,194 |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Expense recognized | $ |  |  | $ (0.8) |  |  |
Restricted Share Cliff Vesting in January 2018 [Member] | One Beacon Insurance Company | Restricted Stock |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) |  |  | 75,950 |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Issued | $ |  |  | $ 1.1 |  |  |
Restricted Stock | Sirius Group | W T M Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Beginning of period |  |  | 3,265 | 3,789 |  |
End of period (4) |  |  | 3,571 | 3,265 | 3,789 |
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan |  |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Forfeited shares |  | 35,000 |  |  |  |
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan | Chief Executive Officer |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) | 300,000 | 630,000 |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Number of installments in which the shares vest | Installment | 2 |  |  |  |  | 4
Forfeited shares |  | 35,000 |  |  |  |
Years performance shares will be forfeited |  | 5 years |  |  |  |
Restricted Stock | OneBeacon | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Beginning of period |  |  | 612,500 | 915,000 | 927,000 |
Issued (in shares) |  |  | 75,950 | 0 | 0 |
Vested (in shares) |  |  | (296,000) | (300,000) | (9,000) |
Assumed forfeitures and cancellations (in shares) |  |  | (9,728) | (2,500) | (3,000) |
End of period (4) |  |  | 382,722 | 612,500 | 915,000 |
Unamortized grant date fair value |  |  |  |  |  |
Beginning balance | $ |  |  | $ 3.5 | $ 6.5 | $ 9.6 |
Issued | $ |  |  | 1.1 | 0.0 | 0.0 |
Vested | $ |  |  | 0.0 |  | 0.0 |
Assumed forfeitures and cancellations | $ |  |  | (0.1) | 0.0 | 0.0 |
Expense recognized | $ |  |  | (2.0) | (3.0) | (3.1) |
Ending balance | $ |  |  | $ 2.5 | $ 3.5 | $ 6.5 |

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