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SCHEDULE I SUMMARY OF INVESTMENTS-OTHER THAN INVESTMENTS IN RELATED PARTIES
 12 Months Ended
Dec. 31, 2020
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
At December 31, 2020

MillionsCostCarrying
Value		Fair
Value
Fixed maturity investments:   
U.S. Government and agency obligations$173.2 $176.3 $176.3 
Debt securities issued by corporations522.8 547.4 547.4 
Municipal obligations244.0 265.0 265.0 
Mortgage and asset-backed securities211.7 218.5 218.5 
Total fixed maturity investments1,151.7 1,207.2 1,207.2 
Short-term investments142.9 142.9 142.9 
Investment in MediaAlpha— 802.2 802.2 
Other long-term investments 767.4 786.8 786.8 
Total investments$2,062.0 $2,939.1 $2,939.1