SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED BALANCE SHEETS (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Assets
Common equity securities, at fair value	$ 650.0	$ 538.4
Other long-term investments	2,150.2	1,998.2
Total assets	9,925.6	8,385.9
Liabilities
Total liabilities	4,794.6	3,824.3
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Total liabilities and equity	9,925.6	8,385.9
White Mountains Insurance Group Ltd.
Assets
Cash	0.1	0.3
Fixed maturity investments, at fair value	70.0	0.0
Common equity securities, at fair value	234.7	147.8
Short-term investments, at fair value	62.4	198.8
Other long-term investments	74.0	160.5
Net receivable due from subsidiaries	118.3	89.1
Other assets	4.6	4.0
Investments in consolidated subsidiaries	3,944.5	3,666.9
Total assets	4,508.6	4,267.4
Liabilities
Other liabilities	24.9	26.9
Total liabilities	24.9	26.9
Total White Mountains’s common shareholders’ equity	4,483.7	4,240.5
Total liabilities and equity	$ 4,508.6	$ 4,267.4