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Segment Information - Assets (Details) - USD ($) $ in Millions | Dec. 31, 2024 | Dec. 31, 2023
--- | --- | ---
Segment reporting information |  |
Total investments | $ 6,477.6 | $ 6,388.7
Total assets | 9,925.6 | 8,385.9
Total liabilities | 4,794.6 | 3,824.3
Total White Mountains’s common shareholders’ equity | 4,483.7 | 4,240.5
Noncontrolling interests | 647.3 | 321.1
Debt | 562.5 | 564.6
Subsidiary - Ark Insurance Holdings Limited |  |
Segment reporting information |  |
Noncontrolling interests | 410.4 | 336.9
Subsidiary - HG Global |  |
Segment reporting information |  |
Noncontrolling interests | (13.4) | 0.8
Kudu |  |
Segment reporting information |  |
Noncontrolling interests | 127.6 | 113.8
Bamboo |  |
Segment reporting information |  |
Noncontrolling interests | 113.6 | 0.0
Other |  |
Segment reporting information |  |
Noncontrolling interests | 9.1 | 9.4
BAM Surplus Notes | Surplus Notes |  |
Segment reporting information |  |
Debt | 300.9 | 322.2
Interest receivable | 194.8 | 174.5
Other Operations |  |
Segment reporting information |  |
Total investments | 1,570.4 | 1,709.1
Total assets | 1,754.2 | 1,875.9
Total liabilities | 181.3 | 141.1
Total White Mountains’s common shareholders’ equity | 1,563.8 | 1,725.4
Noncontrolling interests | 9.1 | 9.4
Debt | 22.0 | 28.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments |  |
Segment reporting information |  |
Total investments | 3,139.7 | 2,671.1
Total assets | 5,299.0 | 4,378.6
Total liabilities | 3,664.8 | 2,876.1
Total White Mountains’s common shareholders’ equity | 1,223.8 | 1,165.6
Noncontrolling interests | 410.4 | 336.9
Debt | 154.5 | 185.5
Financial Guarantee (HG Global) | Operating Segments |  |
Segment reporting information |  |
Total investments | 667.6 | 1,082.9
Total assets | 1,179.4 | 1,172.0
Total liabilities | 464.1 | 531.7
Total White Mountains’s common shareholders’ equity | 728.7 | 779.3
Noncontrolling interests | (13.4) | (139.0)
Dividends payable | 462.1 | 399.8
Debt | 147.4 | 146.9
Asset Management (Kudu) | Operating Segments |  |
Segment reporting information |  |
Total investments | 1,041.9 | 925.6
Total assets | 1,108.4 | 959.4
Total liabilities | 316.7 | 275.4
Total White Mountains’s common shareholders’ equity | 664.1 | 570.2
Noncontrolling interests | 127.6 | 113.8
Debt | 238.6 | 203.8
P&C Insurance Distribution (Bamboo) | Operating Segments |  |
Segment reporting information |  |
Total investments | 58.0 | 0.0
Total assets | 584.6 | 0.0
Total liabilities | 167.7 | 0.0
Total White Mountains’s common shareholders’ equity | 303.3 | 0.0
Noncontrolling interests | $ 113.6 | $ 0.0

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