| Document and Entity Information | Dec. 14, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Dec. 14, 2020 |
| 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 |
| Trading Symbol | WTM |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000776867 |
| Amendment Flag | false |
