SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED BALANCE SHEETS (Details) - USD ($) $ in Millions	Dec. 31, 2025	Dec. 31, 2024
Assets
Common equity securities, at fair value	$ 483.0	$ 650.0
Other long-term investments	2,958.5	2,150.2
Total assets	12,306.5	9,925.6
Liabilities
Total liabilities	6,051.4	4,794.6
Total White Mountains’s common shareholders’ equity	5,425.4	4,483.7
Total liabilities, redeemable noncontrolling interests and equity	12,306.5	9,925.6
White Mountains Insurance Group Ltd.
Assets
Cash	0.4	0.1
Fixed maturity investments, at fair value	35.3	70.0
Common equity securities, at fair value	11.7	234.7
Short-term investments, at fair value	55.1	62.4
Other long-term investments	50.1	74.0
Net receivable due from subsidiaries	0.0	118.3
Other assets	5.3	4.6
Investments in consolidated and unconsolidated subsidiaries	5,330.6	3,943.9
Net assets held for sale	1.4	0.6
Total assets	5,489.9	4,508.6
Liabilities
Net payable due to subsidiaries	32.4	0.0
Other liabilities	32.1	24.9
Total liabilities	64.5	24.9
Total White Mountains’s common shareholders’ equity	5,425.4	4,483.7
Total liabilities, redeemable noncontrolling interests and equity	$ 5,489.9	$ 4,508.6