Held for Sale	3 Months Ended
Mar. 31, 2026
Discontinued Operations and Disposal Groups [Abstract]
Held for Sale	Held for Sale White Mountains recognizes assets and liabilities classified as held for sale at the lower of carrying value on the date the asset is initially classified as held for sale or fair value less costs to sell. At the time of reclassification to held for sale, White Mountains ceases to recognize depreciation and amortization on assets held for sale. White Mountains reported assets and liabilities held for sale related to the pending sale of an Other Operating Business. As of March 31, 2026 and December 31, 2025, White Mountains reported assets and liabilities held for sale related to the pending sale of an Other Operating Business, which included $4.5 million and $5.0 million of short-term investments, cash and other assets as well as $2.9 million and $3.6 million of other liabilities.

Held for Sale

3 Months Ended

Mar. 31, 2026

Discontinued Operations and Disposal Groups [Abstract]

White Mountains recognizes assets and liabilities classified as held for sale at the lower of carrying value on the date the asset is initially classified as held for sale or fair value less costs to sell. At the time of reclassification to held for sale, White Mountains ceases to recognize depreciation and amortization on assets held for sale. White Mountains reported assets and liabilities held for sale related to the pending sale of an Other Operating Business. As of March 31, 2026 and December 31, 2025, White Mountains reported assets and liabilities held for sale related to the pending sale of an Other Operating Business, which included $4.5 million and $5.0 million of short-term investments, cash and other assets as well as $2.9 million and $3.6 million of other liabilities.