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Summary of Significant Accounting Policies (Basis of Presentation) (Details) (USD $)
In Millions, unless otherwise specified
 1 Months Ended 3 Months Ended 12 Months Ended 9 Months Ended 12 Months Ended 12 Months Ended 1 Months Ended 12 Months Ended
Dec. 31, 2011
company
Dec. 31, 2012
state
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
state
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2012
OneBeacon Ltd.
Dec. 31, 2011
OneBeacon Ltd.
Dec. 31, 2012
HG Global
Sep. 30, 2012
White Mountains
Dec. 31, 2012
White Mountains
Dec. 31, 2012
Redeemable preferred stocks
HG Global
Dec. 31, 2012
Common shareholders’ equity
HG Global
Dec. 31, 2011
Tuckerman Fund I
Dec. 31, 2010
Tuckerman Fund I
Dec. 31, 2011
Tuckerman Fund I
Common shareholders’ equity
Dec. 31, 2011
Tuckerman Fund I
Common shareholders’ equity
Basis of Presentation                                              
Ownership percentage in outstanding common shares                         75.20% 75.50%                  
Funds to capitalize HG Global                               $ 600.0 $ 594.5            
Surplus notes                             503.0                
Number of manufacturing companies 2                                            
Ownership interest (as a percent)                             97.30%     97.30% 88.70%        
Ownership interest in VIE (as a percent)                                           94.00% 94.00%
Percentage of par value of policy reinsured   15.00%               15.00%         15.00%                
Number of states   30               30                          
Pre-tax income (loss)   $ (12.0) $ (30.9) $ 12.1 $ 16.8 $ 20.3 $ (11.0) $ 20.8 $ 11.4 $ 14.0 $ (41.5) $ (53.0)               $ (2.2) $ (1.0)