Document and Entity Information - $ / shares	3 Months Ended
	Mar. 31, 2019	May 02, 2019	Dec. 31, 2018
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	Mar. 31, 2019
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		3,181,199
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q1
Entity Emerging Growth Company	false
Entity Small Business	false
Common shares, par value per share (in dollars per share)	$ 1.00		$ 1.00