v3.25.2
Segment Information - Assets (Details) - USD ($)
$ in Millions
Jun. 30, 2025
Dec. 31, 2024
Segment Reporting Information [Line Items]    
Total investments $ 6,798.6 $ 6,477.6
Total assets 11,822.9 9,925.6
Total liabilities 6,482.7 4,794.6
Total White Mountains’s common shareholders’ equity 4,644.5 4,483.7
Noncontrolling interests 695.7 647.3
Other Operations    
Segment Reporting Information [Line Items]    
Total investments 1,438.2 1,570.4
Total assets 1,822.7 1,754.2
Total liabilities 210.5 181.3
Total White Mountains’s common shareholders’ equity 1,572.2 1,563.8
Noncontrolling interests 40.0 9.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 3,411.3 3,139.7
Total assets 6,905.5 5,299.0
Total liabilities 5,173.5 3,664.8
Total White Mountains’s common shareholders’ equity 1,284.6 1,223.8
Noncontrolling interests 447.4 410.4
Financial Guarantee (HG Global) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 713.7 667.6
Total assets 1,244.5 1,179.4
Total liabilities 488.3 464.1
Total White Mountains’s common shareholders’ equity 768.6 728.7
Noncontrolling interests (12.4) (13.4)
Dividends payable 494.2 462.1
Asset Management (Kudu) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 1,169.5 1,041.9
Total assets 1,230.7 1,108.4
Total liabilities 329.2 316.7
Total White Mountains’s common shareholders’ equity 773.9 664.1
Noncontrolling interests 127.6 127.6
Bamboo | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 65.9 58.0
Total assets 619.5 584.6
Total liabilities 281.2 167.7
Total White Mountains’s common shareholders’ equity 245.2 303.3
Noncontrolling interests $ 93.1 $ 113.6