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Employee Share-Based Incentive Compensation Plans (OneBeacon Restricted Shares) (Details) $ in Millions |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Feb. 24, 2016 shares | May 31, 2011 shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares | Dec. 31, 2014 USD ($) shares | May 25, 2011 Installment
Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan |  |  |  |  |  |
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |
Employee Service Share-based Compensation, Total Compensation Cost Not Yet Recognized | $ |  |  | $ 3.9 |  |  |
Defined Contribution Plan, Cost Recognized | $ |  |  | $ 2.1 | $ 0.8 |  |
Percentage of common shares outstanding |  |  | 100.00% |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) |  |  | 233,461 | 226,778 |  |
Restricted Share Cliff Vesting in January 2018 [Member] | One Beacon Insurance Company | Restricted Stock |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) |  |  |  | 75,950 |  |
Restricted Stock | Sirius Group | W T M Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Beginning of period |  |  | 9,205 | 12,477 |  |
End of period (3) |  |  | 5,235 | 9,205 | 12,477 |
Restricted Stock | OneBeacon | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Beginning of period |  |  | 382,722 | 612,500 | 915,000 |
Issued (in shares) |  |  | 170,650 | 75,950 | 0 |
Vested (in shares) |  |  | (157,500) | (296,000) | (300,000) |
Assumed forfeitures and cancellations (in shares) |  |  | 0 | (9,728) | (2,500) |
Employee Service Share-based Compensation Nonvested Awards Forfeited in Period Total Compensation Cost Not yet Recognized | $ |  |  | $ 0.0 | $ (0.1) |  |
End of period (3) |  |  | 395,872 | 382,722 | 612,500 |
Unamortized grant date fair value |  |  |  |  |  |
Beginning balance | $ |  |  | $ 2.5 | $ 3.5 | $ 6.5 |
Issued | $ |  |  | 2.3 | 1.1 | 0.0 |
Vested | $ |  |  | 0.0 |  | 0.0 |
Expense recognized | $ |  |  | (2.7) | (2.0) | (3.0) |
Ending balance | $ |  |  | $ 2.1 | $ 2.5 | $ 3.5 |
Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) | 170,650 |  |  |  |  |
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) |  | 630,000 |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |
Number of installments in which the shares vest | Installment |  |  |  |  |  | 4
Forfeited shares |  | 35,000 |  |  |  |
Years performance shares will be forfeited |  | 5 years |  |  |  |
Vesting in January 2019 [Member] | Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) | 78,150 |  |  |  |  |
Vesting in February 2018 [Member] | Restricted Stock | One Beacon Insurance Company | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
Issued (in shares) | 92,500 |  |  |  |  |
Restricted Share Cliff Vesting in January 2017 [Member] | Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
End of period (3) |  |  | 173,042 |  |  |
Restricted Share Cliff Vesting in January 2018 [Member] | Restricted Stock Units (RSUs) [Member] | OneBeacon | Long Term Incentive Plan |  |  |  |  |  |
Target Performance Share activity |  |  |  |  |  |
End of period (3) |  |  | 214,506 |  |  |

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