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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) - OneBeacon - One Beacon Long Term Incentive Plan $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 24, 2015 shares | Mar. 01, 2012 shares | May. 31, 2011 shares | Sep. 30, 2015 USD ($) shares | Sep. 30, 2014 USD ($) shares | Sep. 30, 2015 USD ($) shares | Sep. 30, 2014 USD ($) shares | Dec. 31, 2014 USD ($) shares | Feb. 28, 2012 Installment | May. 25, 2011 Installment
Restricted Stock |  |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 390,950 | 612,500 | 612,500 | 915,000 | 915,000 |  |
Issued (in shares) | 75,950 | 300,000 |  | 0 | 0 | 75,950 | 0 |  |  |
Vested (in shares) |  |  |  | 0 | 0 | (296,000) | (300,000) |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value | $ |  |  |  |  |  | $ 0.0 |  |  |  |
Assumed forfeitures and cancellations |  |  |  | 0 | 0 | (1,500) | (2,500) |  |  |
Ending of period (in shares) |  |  |  | 390,950 | 612,500 | 390,950 | 612,500 | 612,500 |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |
Unamortized grant date fair value, beginning balance | $ |  |  |  | $ 3.5 | $ 4.9 | $ 3.5 | $ 6.5 | $ 6.5 |  |
Issued | $ |  |  |  | 0.0 | 0.0 | 1.1 | 0.0 |  |  |
Assumed forfeitures and cancellations | $ |  |  |  | 0.0 | 0.0 | 0.0 | 0.0 |  |  |
Expense recognized | $ |  |  |  | (0.5) | (0.7) | (1.6) | (2.3) |  |  |
Unamortized grant date fair value, ending balance | $ |  |  |  | $ 3.0 | $ 4.2 | $ 3.0 | $ 4.2 | $ 3.5 |  |
Number of Installments in which the Shares Vest | Installment |  |  |  |  |  |  |  |  | 2 |
Restricted Stock | Chief Executive Officer |  |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |  |
Issued (in shares) |  |  | 630,000 |  |  |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |
Number of Installments in which the Shares Vest | Installment |  |  |  |  |  |  |  |  |  | 4
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Reduction in the next five years |  |  | 35,000 |  |  |  |  |  |  |
Performance Share |  |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 462,147 | 512,938 | 517,470 | 493,421 | 493,421 |  |
Assumed forfeitures and cancellations |  |  |  | 0 | 0 | (28,920) | (4,145) |  |  |
Ending of period (in shares) |  |  |  | 462,147 | 512,938 | 462,147 | 512,938 | 517,470 |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations | $ |  |  |  | $ 0.0 | $ 0.0 | $ 0.0 | $ 0.0 |  |  |
Expense recognized | $ |  |  |  | (0.3) | $ 1.1 | 0.2 | $ 2.5 |  |  |
Unamortized grant date fair value, ending balance | $ |  |  |  | $ 2.0 |  | $ 2.0 |  |  |  |
Performance Share | Chief Executive Officer |  |  |  |  |  |  |  |  |  |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |  |
Annual reduction in performance shares |  |  | 35,000 |  |  |  |  |  |  |
Years Performance Shares will be Forfeited |  |  | 5 years |  |  |  |  |  |  |

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