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Summary of Significant Accounting Policies (Details 2) (USD $)
In Millions, except Share data, unless otherwise specified
 3 Months Ended 6 Months Ended 1 Months Ended
Jun. 30, 2012
Jun. 30, 2011
Jun. 30, 2012
Jun. 30, 2011
Jan. 02, 2012
Dec. 31, 2011
Jun. 30, 2012
OneBeacon Ltd.
Dec. 31, 2011
OneBeacon Ltd.
May 31, 2007
Sirius Group
Non-cumulative perpetual preference shares
May 24, 2007
Sirius Group
Non-cumulative perpetual preference shares
Jun. 30, 2012
White Mountains consolidated limited partnerships
Dec. 31, 2011
White Mountains consolidated limited partnerships
Jun. 30, 2012
A.W.G. Dewar Inc
Dec. 31, 2011
A.W.G. Dewar Inc
Jun. 30, 2012
Hamer and Bri-Mar
Jun. 30, 2012
Passage 2 Health Limited
Dec. 31, 2011
Passage 2 Health Limited
Noncontrolling Interest                                  
Noncontrolling equity percentage in OneBeacon Ltd.             24.80% 24.50%                  
Non-cumulative perpetual preference shares issued                 250,000,000                
Non-cumulative perpetual preference shares per share liquidation preference (in dollars per share)                   $ 1,000              
Proceeds from non-cumulative perpetual preference shares                 $ 245.7                
Non-cumulative perpetual preference shares issuance cost and commission                   4.3              
Noncontrolling equity interest 61.6   61.6     57.1         54.7 54.2 2.6 2.3 3.9 0.4 0.6
Deferred acquisition costs 200.0   200.0   5.6 187.0                      
Deferred policy acquisition costs, amortization expense $ 2.0 $ 2.5 $ 4.4 $ 4.3