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Employee Share-Based Incentive Compensation Plans (Details 5) (USD $)
 3 Months Ended 1 Months Ended 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2011
Non-Qualified Options
Nov. 30, 2006
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Mar. 31, 2012
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Y
Mar. 31, 2011
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Nov. 16, 2010
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
May 27, 2008
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Mar. 31, 2008
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Share-based compensation arrangement by share-based payment award                  
Granted (in shares)       1,420,000          
Options vesting period (in years)         Third, fourth and fifth anniversaries        
Expiration period of award (in years)         P5Y6M        
Expected volatility (as a percent)         30.00%        
Risk-free rate (as a percent)         4.60%        
Forfeiture (as a percent)         5.00%        
Expected dividend yield (as a percent)         3.40%        
Expected life (in years)         5.5        
Initial exercise price (in dollars per share)         $ 30.00        
Amended exercise price of option (in dollars per share)             $ 25.47 $ 27.97  
OneBeacon special dividend (in dollars per share)             $ 2.50   $ 2.03
Target options outstanding                  
Opening balance - outstanding Options (in shares)         740,870 768,652      
Ending balance - outstanding Options (in shares)         740,870 768,652      
Expense Amortized                  
Accrued expense at the beginning of period $ 90,000,000 $ 187,900,000     $ 5,000,000 $ 4,500,000      
Expense recognized     100,000     100,000      
Accrued expense at the end of the period $ 90,000,000 $ 187,900,000     $ 5,000,000 $ 4,600,000