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OneBeacon Pension Plan (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2016 | Sep. 30, 2015 | Sep. 30, 2016 | Sep. 30, 2015
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Other Income | $ 34.8 | $ 38.3 | $ 110.8 | $ 114.4
Pension Plan [Member] |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 12.6 |  | 12.6 |
Transfer of assets | 47.1 |  | 47.1 |
OneBeacon [Member] |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Other Income | 1.8 | $ 3.9 | $ 3.5 | $ (0.4)
OneBeacon [Member] | Pension Plan [Member] |  |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |  |
Other Income | $ 3.2 |  |  |

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