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Subsequent Event (Notes)	12 Months Ended
Subsequent Event (Notes)	Dec. 31, 2012
Subsequent Events [Abstract]
Subsequent Event	Subsequent Event Sale of Essentia On January 2, 2013, OneBeacon completed the sale of Essentia Insurance Company (“Essentia”), an indirect wholly owned subsidiary that wrote policies for the Hagerty collector car and boat business, to Markel Corporation. OneBeacon will recognize a pre-tax gain on sale of approximately $23 million ($15 million after tax) in the first quarter of 2013. The Hagerty business associated with this agreement generated net written premiums of approximately $179.7 million, $166.6 million and $153 million (8.0% of WhiteMountains’ consolidated written premiums for all periods), for the years ended December 31, 2012, 2011 and 2010.