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Document and Entity Information
 6 Months Ended
Jun. 30, 2012
Jul. 26, 2012
Document and Entity Information [Abstract]    
Entity Registrant Name WHITE MOUNTAINS INSURANCE GROUP LTD  
Entity Central Index Key 0000776867  
Document Type 10-Q  
Document Period End Date Jun. 30, 2012  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Entity Well-known Seasoned Issuer Yes  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Entity Filer Category Large Accelerated Filer  
Entity Common Stock, Shares Outstanding   6,584,718
Document Fiscal Year Focus 2012  
Document Fiscal Period Focus Q2