| Document and Entity Information Document and Entity Information - USD ($) | 12 Months Ended | ||
| Dec. 31, 2020 | Feb. 24, 2021 | Jun. 30, 2020 | |
| Cover [Abstract] | |||
| Document Type | 10-K/A | ||
| Document Annual Report | true | ||
| Document Period End Date | Dec. 31, 2020 | ||
| 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 | ||
| 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 | $ 2,685,847,299 | ||
| Entity Common Stock, Shares Outstanding | 3,095,829 | ||
| Entity Central Index Key | 0000776867 | ||
| Amendment Flag | true | ||
| Current Fiscal Year End Date | --12-31 | ||
| Document Fiscal Year Focus | 2020 | ||
| Document Fiscal Period Focus | FY | ||
| ICFR Auditor Attestation Flag | true |
