Document and Entity Information - shares	6 Months Ended
	Jun. 30, 2016	Jul. 31, 2016
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, 2016
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		4,930,938
Document Fiscal Year Focus	2016
Document Fiscal Period Focus	Q2