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Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 28, 2020
Jun. 30, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
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 80 South Main Street    
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    
Trading Symbol WTM    
Security Exchange Name NYSE    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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 Public Float     $ 3,169,884,560
Entity Common Stock, Shares Outstanding   3,193,509  
Documents Incorporated by Reference Portions of the Registrant’s Definitive Proxy Statement to be filed with the Securities and Exchange Commission (“SEC”) pursuant to Regulation 14A under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), relating to the Registrant’s Annual General Meeting of Members scheduled to be held May 21, 2020 are incorporated by reference into Part III of this Form 10-K. With the exception of the portions of the Proxy Statement specifically incorporated herein by reference, the Proxy Statement is not deemed to be filed as part of this Form 10-K.    
Entity Central Index Key 0000776867    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY