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Retirement and Postretirement Plans (Qualifed, Non-qualified, and Accumulated Benefit Obligations) (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Information for the Non-qualified Plan, which had accumulated benefit obligations in excess of plan assets    
Projected benefit obligation $ 27.6 $ 26.5
Accumulated benefit obligation 27.6 26.5
Fair value of plan assets 0 0
Information for the Qualified Plan, which had accumulated benefit obligations less than plan assets    
Projected benefit obligation 91.9 86.5
Accumulated benefit obligation 91.9 86.5
Fair value of plan net assets 124.7 [1] 120.8 [1]
Amounts recognized in accumulated other comprehensive income (loss) on a pre-tax basis noncontrolling interest    
Accumulated other comprehensive (loss) income beginning balance (16.7) 0.5
Increase (decrease) in accumulated other comprehensive income (loss):    
Amortization of net actuarial losses recognized during the year 0.8 0.5
Net actuarial (losses) gains occurring during the year (5.9) [2] (18.2) [2]
Other adjustments 0.6 0.5
Accumulated other comprehensive loss ending balance (21.2) (16.7)
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 $ 0.9  
[1] Includes receivables related to securities sold, interest and dividends as well as payables related to securities purchased.
[2] Net actuarial losses resulted from a decrease in investment returns on plan assets in the year ended December 31, 2011 and changes in assumptions in estimating the projected benefit obligation in the years ended December 31, 2012 and 2011