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Employee Share-Based Incentive Compensation Plans (Details) (USD $)
 3 Months Ended 1 Months Ended 3 Months Ended 12 Months Ended 3 Months Ended
Mar. 31, 2012
Dec. 31, 2011
Mar. 31, 2012
Performance shares
Performance cycle 2009-2011
Mar. 31, 2011
Non-Qualified Options
Mar. 31, 2012
WTM Incentive Plan and WTM Phantom Share Plans
Performance shares
Mar. 31, 2011
WTM Incentive Plan and WTM Phantom Share Plans
Performance shares
Mar. 31, 2012
WTM Incentive Plan and WTM Phantom Share Plans
Performance shares
Performance cycle 2009-2011
May 31, 2011
OneBeacon Long-Term Incentive Plan
Performance shares
Mar. 31, 2012
OneBeacon Long-Term Incentive Plan
Performance shares
Mar. 31, 2011
OneBeacon Long-Term Incentive Plan
Performance shares
Dec. 31, 2011
OneBeacon Long-Term Incentive Plan
Performance shares
Performance cycle 2008-2010
Mar. 31, 2012
OneBeacon Long-Term Incentive Plan
Performance shares
Performance cycle 2009-2011
Mar. 31, 2012
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Mar. 31, 2011
OneBeacon Long-Term Incentive Plan
Non-Qualified Options
Share-based compensation arrangement by share-based payment award                            
Vesting period (in years)         3 years       3 years       Third, fourth and fifth anniversaries  
Target Performance Share activity                            
Beginning of period (in shares)         150,064 163,184     642,667 1,464,295        
Shares paid or expired (in shares)         (68,357) (51,131)     (249,733) (936,150)        
New grants (in shares)         35,932 37,425     181,290 194,900        
Assumed forfeitures and cancellations         811 342   35,000 (4,479) (4,873)        
Ending of period (in shares)         118,450 149,820     569,745 718,172        
Expense Amortized                            
Accrued expense at the beginning of period $ 90,000,000 $ 187,900,000     $ 66,100,000 $ 29,400,000     $ 9,700,000 $ 18,500,000     $ 5,000,000 $ 4,500,000
Payments and deferrals         (48,400,000)       (7,500,000) (10,500,000)        
Assumed forfeitures and cancellations         1,000,000 (100,000)                
Expense (income) recognized       100,000 8,700,000 6,300,000     1,000,000 400,000       100,000
Accrued expense at the end of the period 90,000,000 187,900,000     27,400,000 35,600,000     3,200,000 8,400,000     5,000,000 4,600,000
Percentage of payout for minimum growth target percentage             147.00%              
Percentage of payout for maximum growth target percentage             155.00%              
Percentage of target achieved by former employees as a result of the Commercial Lines and Personal Lines Transactions                     68.50% 138.60%    
Shares issued during the period related to performance shares earned     9,577                      
Target vesting percentage of performance shares         100.00%       100.00%          
Additional compensation cost that would be recognized if all outstanding performance shares vested         $ 34,500,000       $ 4,600,000