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Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest      
Accumulated other comprehensive (loss) income beginning balance $ (8.0) $ (8.0) $ 10.5
Increase (decrease) in accumulated other comprehensive income (loss):      
Amortization of net actuarial gains recognized during the year   1.3 0.3
Net actuarial (losses) gains occurring during the year   (1.3) (18.8)
Accumulated other comprehensive loss ending balance   (8.0) (8.0)
Other Postretirement Benefit Plan, Defined Benefit [Member]      
Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets      
Projected benefit obligation   25.5 27.1
Accumulated benefit obligation   25.5 27.1
Fair value of plan assets   0.0 0.0
Qualified Plan [Member]      
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets      
Projected benefit obligation   84.0 92.6
Accumulated benefit obligation   84.0 92.6
Fair value of plan net assets   $ 139.8 $ 146.0
Scenario, Forecast [Member] | OneBeacon Ltd.      
Increase (decrease) in accumulated other comprehensive income (loss):      
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost during 2012 $ 1.1