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Derivatives (Pre-Tax Operating Results of WM Life Re) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2012
Sep. 30, 2012
Jun. 30, 2012
Mar. 31, 2012
Dec. 31, 2011
Sep. 30, 2011
Jun. 30, 2011
Mar. 31, 2011
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Pre-tax operating results of WM Life Re                      
Change in fair value of variable annuity liability, included in other revenues                   $ 7.2 $ 47.7
Total revenues 566.4 697.4 542.3 629.7 593.4 501.7 552.4 525.6 2,435.7 2,173.1 2,333.7
General and administrative expenses                 (182.2) (175.3) (155.0)
Pre-tax income (50.8) 135.1 37.6 140.9 98.4 1.2 35.8 (37.4) 262.8 98.0 189.3
White Mountains Life Re
                      
Pre-tax operating results of WM Life Re                      
Fees, included in other revenues                 31.8 32.5 30.2
Change in fair value of variable annuity liability, included in other revenues                 312.8 (156.5) (223.5)
Change in fair value of derivatives, included in other revenues                 (339.0) 92.9 127.0
Foreign exchange, included in other revenues                 (30.3) 15.1 21.4
Other investment income and gains (losses)                 2.5 (0.9) (0.9)
Total revenues                 (22.2) (16.9) (45.8)
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses                 14.2 (1.8) (6.0)
Death benefit claims paid, included in general and administrative expenses                 (5.7) (3.8) (2.7)
General and administrative expenses                 (5.2) (4.7) (6.3)
Pre-tax income                 $ (18.9) $ (27.2) $ (60.8)