Document and Entity Information (USD $)	12 Months Ended
	Dec. 31, 2011	Feb. 27, 2012	Jun. 30, 2011
Document and Entity Information
Entity Registrant Name	WHITE MOUNTAINS INSURANCE GROUP LTD
Entity Central Index Key	0000776867
Document Type	10-K
Document Period End Date	Dec. 31, 2011
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 Public Float			$ 2,888,052,532
Entity Common Stock, Shares Outstanding		7,454,585
Document Fiscal Year Focus	2011
Document Fiscal Period Focus	FY