v3.25.2
Cover Page - shares
 6 Months Ended
Jun. 30, 2025
Aug. 01, 2025
Document Information [Line Items]    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2025  
Document Transition Report false  
Entity File Number 1-8993  
Entity Registrant Name WHITE MOUNTAINS INSURANCE GROUP, LTD  
Entity Incorporation, State or Country Code D0  
Entity Tax Identification Number 94-2708455  
Entity Address, Address Line One 23 South Main Street, Suite 3B  
Entity Address, City or Town Hanover,  
Entity Address, State or Province NH  
Entity Address, Postal Zip Code 03755-2053  
City Area Code 603  
Local Phone Number 640-2200  
Title of 12(b) Security Common Shares, par value $1.00 per share  
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Filer Category Large Accelerated Filer  
Entity Small Business false  
Entity Emerging Growth Company false  
Entity Shell Company false  
Entity Common Stock, Shares Outstanding (in shares)   2,575,161
Entity Central Index Key 0000776867  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Amendment Flag false  
New York Stock Exchange    
Document Information [Line Items]    
Trading Symbol WTM  
Security Exchange Name NYSE