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SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES | 12 Months Ended
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Dec. 31, 2025
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Abstract] |
SCHEDULE I - SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES | WHITE MOUNTAINS INSURANCE GROUP, LTD. SUMMARY OF INVESTMENTS—OTHER THAN INVESTMENTS IN RELATED PARTIES As of December 31, 2025
Millions |  | Cost or Amortized Cost |  | Carrying Value |  | Fair Value
Fixed maturity investments: |  |  |  |  |  |
U.S. Government and agency obligations |  | $ | 459.7 |  |  | $ | 460.3 |  |  | $ | 460.3 |
Debt securities issued by corporations |  | 1,534.9 |  |  | 1,537.7 |  |  | 1,537.7 |
Mortgage and asset-backed securities |  | 405.1 |  |  | 393.0 |  |  | 393.0 |
Collateralized loan obligations |  | 343.9 |  |  | 347.0 |  |  | 347.0 |
Foreign government and agency obligations |  | 31.2 |  |  | 32.5 |  |  | 32.5 |
Total fixed maturity investments |  | 2,774.8 |  |  | 2,770.5 |  |  | 2,770.5 |
Short-term investments |  | 1,881.2 |  |  | 1,881.7 |  |  | 1,881.7 |
Investment in MediaAlpha (1) |  | 59.2 |  |  | 231.2 |  |  | 231.2 |
Common equity securities: |  |  |  |  |  |
Industrial, Miscellaneous and Other |  | 353.7 |  |  | 452.3 |  |  | 452.3 |
Exchange traded funds |  | 30.6 |  |  | 30.7 |  |  | 30.7 |
Total common equity securities |  | 384.3 |  |  | 483.0 |  |  | 483.0 |
Other long-term investments (2) |  | 2,378.8 |  |  | 2,958.5 |  |  | 2,958.5 |
Total investments |  | $ | 7,478.3 |  |  | $ | 8,324.9 |  |  | $ | 8,324.9 |
(1) Under GAAP, equity method eligible investments are considered related parties. White Mountains’s Investment in MediaAlpha is equity method eligible. See Note 17 — “Equity Method Eligible Investments.” (2) Includes equity method eligible investments with a cost and carrying value of $1,727.3 and $2,375.1.

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- Definition The entire disclosure of information about investment held by insurance company, excluding investment in related party. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Name Regulation S-X (SX) -Number 210 -Section 12 -Subsection 15 -Publisher SEC Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 7 -Subparagraph (SX 210.12-15) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-7 + Details
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