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Investments in Unconsolidated Affiliates (Investments in Symetra) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Dec. 31, 2011
Changes in carrying value of investment in Symetra      
Carrying value at the beginning of the period $ 387.9    
Equity in earnings 9.2 10.2  
Carrying value at the end of the period 383.6    
Symetra
      
Changes in carrying value of investment in Symetra      
Carrying value at the beginning of the period 381.5 273.6  
Equity in earnings 9.8 [1],[2] 10.9 [1],[2]  
Equity in net unrealized losses from Symetra's fixed maturity portfolio (8.7) [3] (1.2) [3]  
Dividends received (1.4) (1.2)  
Increase (decrease) in value of warrants 3.7 (11.1)  
Carrying value at the end of the period 377.5 [4],[5] 293.2 [4],[5] 273.6
Tax on equity in earnings 0.8 0.7  
Equity in net unrealized losses increase relating to amortization of common share impairment     136.6
Tax on equity in net unrealized gains (losses) 0.7 0  
Aggregate value of White Mountains common shares of Symetra at quoted market price 233.3    
Symetra | Common shareholders' equity.
      
Changes in carrying value of investment in Symetra      
Carrying value at the beginning of the period 351.2 261.0  
Equity in earnings 9.8 [1],[2] 10.9 [1],[2]  
Equity in net unrealized losses from Symetra's fixed maturity portfolio (8.7) [3] (1.2) [3]  
Dividends received (1.4) (1.2)  
Increase (decrease) in value of warrants 0 0  
Carrying value at the end of the period 350.9 [4],[5] 269.5 [4],[5]  
Impairment of Shares Change in Equity in Earnings 0.7 1.1  
Equity in net unrealized losses increase relating to amortization of common share impairment 2.7 3.2  
Symetra | Warrants
      
Changes in carrying value of investment in Symetra      
Carrying value at the beginning of the period 30.3 12.6  
Equity in earnings 0 [1],[2] 0 [1],[2]  
Equity in net unrealized losses from Symetra's fixed maturity portfolio 0 [3] 0 [3]  
Dividends received 0 0  
Increase (decrease) in value of warrants 3.7 (11.1)  
Carrying value at the end of the period 26.6 [4],[5] 23.7 [4],[5]  
Cash dividends received on warrants $ 0.8    
[1] Equity in earnings includes $0.7 and $1.1 increase relating to the pre-tax amortization of Symetra common share impairment from March 31, 2013 and 2012.
[2] Equity in earnings excludes tax expense of $0.8 and $0.7.
[3] Net unrealized gains includes $2.7 and $3.2 increase relating to the pre-tax amortization of Symetra common share impairment from March 31, 2013 and 2012.
[4] Includes White Mountains’ equity in net unrealized gains from Symetra’s fixed maturity portfolio of $(8.7) and $(1.2) as of March 31, 2013 and 2012, which exclude tax benefit of $0.7 and $0.0.
[5] The aggregate value of White Mountains’ investment in common shares of Symetra was $233.3 based upon the quoted market price of $13.41 per share at March 31, 2013.