Document and Entity Information - $ / shares	6 Months Ended
	Jun. 30, 2018	Aug. 06, 2018	Dec. 31, 2017
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, 2018
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		3,180,358
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	Q2
Common shares, par value per share (in dollars per share)	$ 1.00		$ 1.00