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Employee Share-Based Incentive Compensation Plans (Details 6) (USD $)
 3 Months Ended 1 Months Ended 3 Months Ended
Mar. 31, 2012
OneBeacon Restricted Stock
Mar. 31, 2008
OneBeacon Long-Term Incentive Plan
Restricted Stock Units, RSU
Mar. 31, 2012
OneBeacon Long-Term Incentive Plan
OneBeacon Restricted Stock
Mar. 01, 2012
OneBeacon Long-Term Incentive Plan
OneBeacon Restricted Stock
installment
May 31, 2011
OneBeacon Long-Term Incentive Plan
OneBeacon Restricted Stock
Chairman and CEO
Y
May 25, 2011
OneBeacon Long-Term Incentive Plan
OneBeacon Restricted Stock
Chairman and CEO
installment
Mar. 31, 2012
OneBeacon Long-Term Incentive Plan
Performance shares
Mar. 31, 2011
OneBeacon Long-Term Incentive Plan
Performance shares
Share-based compensation arrangement by share-based payment award                
Total number of performance share grants to be reduced         35,000      
Number of installments in which the shares vest       2   4    
Activity in plan                
Beginning of period (in shares) 630,000           642,667 1,464,295
Issued (in shares) 300,000   300,000   630,000      
Ending of period (in shares) 930,000           569,745 718,172
Unamortized grant date fair value                
Unamortized grant date fair value, beginning balance $ 7,700,000              
Issued 4,600,000              
Expense recognized (400,000)           1,000,000 400,000
Unamortized grant date fair value, ending balance 11,900,000              
New grants (in shares)   116,270         181,290 194,900
Proportion of award vests on each of Nov 9, 2009, 2010 and 2011   0.33            
Minimum percentage of growth in book value per share for vesting each tranche of units   4.00%            
Expense recognized $ (400,000)           $ 1,000,000 $ 400,000
Period in which performance share awards will be reduced by a similar number of shares (in years)         5