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Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements   $ 3.0 $ 0.0 $ 0.0
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest        
Accumulated other comprehensive loss beginning balance $ (6.5) (8.0) (8.0)  
Increase (decrease) in accumulated other comprehensive loss:        
Amortization of net actuarial gains recognized during the year   1.3 1.3  
Net actuarial (losses) gains occurring during the year   3.2 (1.3)  
Other Comprehensive Income Defined Benefit Plans Other Adjustments before Tax   (3.0) 0.0  
Accumulated other comprehensive loss ending balance   (6.5) (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   23.8 25.5  
Accumulated benefit obligation   23.8 25.5  
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   0.0 84.0  
Accumulated benefit obligation   0.0 84.0  
Fair value of plan net assets   13.0 $ 139.8  
Scenario, Forecast [Member] | OneBeacon Ltd.        
Increase (decrease) in accumulated other comprehensive loss:        
Amount expected to be amortized from accumulated other comprehensive income (loss) into net periodic benefit cost during 2012 $ 0.9      
Qualified Plan [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Defined Benefit Plan, Recognized Net Gain (Loss) Due to Settlements   $ 3.0