Document and Entity Information	6 Months Ended
	Jun. 30, 2013	Jul. 30, 2013
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, 2013
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		6,176,489
Document Fiscal Year Focus	2013
Document Fiscal Period Focus	Q2
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No