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Employee Share-Based Incentive Compensation Plans (OneBeacon - Restricted Shares) (Details) $ in Millions |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Feb. 24, 2016 shares | Feb. 24, 2015 shares | May 31, 2011 shares | Sep. 30, 2016 USD ($) shares | Sep. 30, 2015 USD ($) shares | Jun. 30, 2015 USD ($) shares | Sep. 30, 2016 USD ($) shares | Sep. 30, 2015 USD ($) shares | May 25, 2011 Installment
Share-based compensation arrangement by share-based payment award |  |  |  |  |  |  |  |  |
Equity in earnings of unconsolidated affiliates, net of tax | $ |  |  |  | $ 0.0 | $ 3.9 |  | $ 0.0 | $ 18.0 |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 395,872 | 390,950 | 612,500 | 382,722 | 612,500 |
Issued (in shares) | 170,650 | 75,950 |  | 0 | 0 |  | 170,650 | 75,950 |
Vested (in shares) |  |  |  | 0 | 0 |  | (157,500) | (296,000) |
Assumed forfeitures and cancellations |  |  |  | 0 | 0 |  | 0 | (1,500) |
Ending of period (in shares) |  |  |  | 395,872 | 390,950 | 390,950 | 395,872 | 390,950 |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |
Unamortized grant date fair value, beginning balance | $ |  |  |  | $ 3.5 | $ 3.5 | $ 3.5 | $ 2.5 | $ 3.5 |
Issued | $ |  |  |  | 0.0 | 0.0 |  | 2.3 | 1.1 |
Vested | $ |  |  |  |  |  |  | 0.0 |  |
Assumed forfeitures and cancellations | $ |  |  |  | 0.0 | 0.0 |  | 0.0 | 0.0 |
Expense recognized | $ |  |  |  | (0.7) | (0.5) |  | (2.0) | (1.6) |
Unamortized grant date fair value, ending balance | $ |  |  |  | $ 2.8 | $ 3.0 | $ 3.5 | $ 2.8 | $ 3.0 |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | 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 |  |  |  |  |  |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Performance Share |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |
Beginning of period (in shares) |  |  |  | 441,206 | 462,147 | 517,470 | 449,435 | 517,470 |
Assumed forfeitures and cancellations |  |  |  | 0 | 0 |  | (4,079) | (28,920) |
Ending of period (in shares) |  |  |  | 441,206 | 462,147 | 462,147 | 441,206 | 462,147 |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |
Assumed forfeitures and cancellations | $ |  |  |  | $ 0.0 | $ 0.0 |  | $ 0.0 | $ 0.0 |
Expense recognized | $ |  |  |  | (0.3) | $ 0.3 |  | (0.6) | $ (0.2) |
Unamortized grant date fair value, ending balance | $ |  |  |  | $ 2.3 |  |  | $ 2.3 |  |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | 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 |  |  |  |  |  |
OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock Units (RSUs) [Member] |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |
Issued (in shares) |  |  |  |  |  |  | 233,461 | 215,698 |
Ending of period (in shares) |  |  |  | 219,112 | 177,409 |  | 219,112 | 177,409 |
Unamortized grant date fair value |  |  |  |  |  |  |  |  |
Expense recognized | $ |  |  |  | $ (0.5) | $ (0.2) |  | $ (1.4) | $ (0.6) |
Restricted Share Vesting in February 2018 [Member] | OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |
Issued (in shares) | 92,500 |  |  |  |  |  |  |  |
Restricted Share Vesting in February 2019 [Member] | OneBeacon [Member] | One Beacon Long Term Incentive Plan | Restricted Stock |  |  |  |  |  |  |  |  |
Target Restricted Share Activity |  |  |  |  |  |  |  |  |
Issued (in shares) | 78,150 |  |  |  |  |  |  |  |

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