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Employee Share-Based Incentive Compensation Plans (WTM Performance Shares) (Details) (USD $)
 12 Months Ended 9 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2012
Performance Share
multiplier
Dec. 31, 2012
W T M Incentive and Phantom Plan
Performance Share
Dec. 31, 2011
W T M Incentive and Phantom Plan
Performance Share
Dec. 31, 2010
W T M Incentive and Phantom Plan
Performance Share
Dec. 31, 2012
W T M Incentive and Phantom Plan
Performance Cycle From 2010 to 2012
Dec. 31, 2012
One Beacon Employee Stock Ownership Plan
Dec. 31, 2012
Sirius Groups Profit Sharing Plan
Dec. 31, 2012
W T M Incentive and Phantom Plan
Performance Share
Sep. 30, 2012
Performance Cycle From 2009 to 2011
Performance Share
Dec. 31, 2012
Performance Cycle From 2009 to 2011
W T M Incentive and Phantom Plan
Performance Share
Dec. 31, 2012
One Beacon Segment
Share-based compensation arrangement by share-based payment award                          
Vesting period (in years)                   3 years      
Annual base contribution provided to participants expressed as percentage of salary               3.00%          
Social security wage base amount maximum               $ 110,100          
Variable contribution provided to participants not eligible to certain benefits expressed as percentage of salary               6.00%          
Employee stock ownership Plan (ESOP), salary amount maximum                         250,000
Variable contribution provided to participants expressed as percentage of salary                 7.00%        
Multiplier for determining payout, low end of range     0                    
Multiplier for determining payout, high end of range     2                    
Performance cycle period     3 years                    
Target Performance Share activity                          
Beginning of period (in shares)       150,064 163,184 172,454              
Shares paid or expired (in shares)       (68,357) [1] (51,131) [1] (49,127) [1]              
New grants (in shares)       38,432 37,675 47,880              
Assumed forfeitures and cancellations (in shares)       (782) [2] 336 [2] (8,023) [2]              
Ending of period (in shares)       119,357 150,064 163,184              
Accrued Expense                          
Accrued expense at the beginning of period 159,000,000 187,900,000   66,100,000 29,400,000 12,200,000              
Payments and deferrals       (58,300,000) [1] 0 [1] 0 [1]              
Assumed forfeitures and cancelations       600,000 [2] (900,000) [2] (700,000) [2]              
Expense recognized       21,000,000 37,600,000 17,900,000              
Accrued expense at the end of the period 159,000,000 187,900,000   29,400,000 66,100,000 29,400,000              
Performance goal percentage for minimum payout                       147.00%  
Payout for maximum growth target percentage                       155.00%  
Partial Payout for 2010 - 2012 cycle             9.9            
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested in Period                     9,577    
Additional compensation cost that would be recognized if all outstanding performance shares vested       $ 21,100,000                  
[1] WTM performance share payments in 2012 for the 2009-2011 performance cycle ranged from 147% to 155% of target. At December 31, 2012, White Mountains paid $9.9 as a partial payment for the 2010-2012 performance cycle. There were no payments made in 2011 and 2010 for the 2008-2010 and 2007-2009 performance cycles; those performance shares did not meet the threshold performance goals and expired.
[2] Amounts include changes in assumed forfeitures, as required under GAAP.