| 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 |
