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Document and Entity Information Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 28, 2020
Jun. 30, 2019
Cover page.      
Document Type 10-K/A    
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  
Amendment Description This Amendment No 1. on Form 10-K/A (the “Amendment”) is filed by White Mountains Insurance Group, Ltd. (“White Mountains” or the “Company”) to amend its Annual Report on Form 10-K for the year ended December 31, 2019 (the “2019 Form 10-K”) filed on March 2, 2020. The purpose of the Amendment is to amend Part IV, Item 15 of the 2019 Form 10-K to include the audited consolidated financial statements of QL Holdings, LLC (“MediaAlpha”) as of December 31, 2019 and for the period from February 26, 2019 through December 31, 2019, and to include the consolidated financial statements of MediaAlpha as of and for the years ended December 31, 2019 and 2018 (not covered by the Independent Auditor's Report). MediaAlpha became an equity method eligible investee on February 26, 2019 and was significant under the income test of Rule 3-09 for the period from February 26, 2019 through December 31, 2019. No other items of the 2019 Form 10-K are amended in the Form 10-K/A.    
Entity Central Index Key 0000776867    
Amendment Flag true    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY