v3.25.1
Document and Entity Information
May 07, 2025
Cover [Abstract]  
Document Type 8-K
Entity Registrant Name WHITE MOUNTAINS INSURANCE GROUP, LTD.
Entity Incorporation, State or Country Code D0
Entity File Number 1-8993
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
City Area Code 603
Local Phone Number 640-2200
Written Communications false
Soliciting Material false
Pre-commencement Tender Offer false
Pre-commencement Issuer Tender Offer false
Title of 12(b) Security Common Shares, par value $1.00 per share
Trading Symbol WTM
Security Exchange Name NYSE
Entity Emerging Growth Company false
Entity Central Index Key 0000776867
Amendment Flag false