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Retirement and Postretirement Plans (Benefit Obligations) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]      
Defined Benefit Plan, Settlements, Benefit Obligation $ (1.8) $ 0.0  
Change in projected benefit obligation:      
Projected benefit obligation at beginning of year 119.7 102.9  
Service cost 0.9 0.6 $ 0.8
Interest cost 4.6 4.7 4.2
Special termination benefits expense 0.0 0.3 0.3
Assumption changes (3.5) 17.5  
Actuarial loss 0.9 1.1  
Benefits and expenses paid with plan assets (13.7) (5.2)  
Benefits paid directly by OneBeacon (2.2) (2.2)  
Projected benefit obligation at end of year 109.5 119.7 102.9
Change in plan assets:      
Fair value of plan assets at beginning of year 146.0 142.8  
Actual return on plan assets 7.5 8.4  
Benefits and expenses paid with plan assets (13.7) (5.2)  
Fair value of plan assets at end of year 139.8 146.0 $ 142.8
Funded status at end of year 30.3 26.3  
Amounts recognized in the financial statements      
Net assets of the Qualified Plan recorded in other assets 55.8 53.4  
Net liabilities of the Non-qualified Plan recorded in other liabilities (25.5) (27.1)  
Defined Benefit Plan, Amounts Recognized in Balance Sheet 30.3 26.3  
Defined Benefit Plan, Other Costs (4.6) 0.0  
Qualified Plan [Member]      
Change in plan assets:      
Fair value of plan assets at beginning of year 143.2    
Fair value of plan assets at end of year 139.4 $ 143.2  
Amounts recognized in the financial statements      
Net assets of the Qualified Plan recorded in other assets 55.8    
Other Postretirement Benefit Plan, Defined Benefit [Member]      
Amounts recognized in the financial statements      
Net liabilities of the Non-qualified Plan recorded in other liabilities (25.5)    
Other Postretirement Benefit Plan, Defined Benefit [Member] | OneBeacon Ltd.      
Change in plan assets:      
Amount set aside in irrevocable rabbi trust $ 18.6