v3.25.1
Segment Information - Assets (Details) - USD ($)
$ in Millions
Mar. 31, 2025
Dec. 31, 2024
Segment Reporting Information [Line Items]    
Total investments $ 6,551.3 $ 6,477.6
Total assets 11,004.9 9,925.6
Total liabilities 5,865.2 4,794.6
Total White Mountains’s common shareholders’ equity 4,509.6 4,483.7
Noncontrolling interests 630.1 647.3
Other Operations    
Segment Reporting Information [Line Items]    
Total investments 1,447.2 1,570.4
Total assets 1,638.0 1,754.2
Total liabilities 130.5 181.3
Total White Mountains’s common shareholders’ equity 1,498.4 1,563.8
Noncontrolling interests 9.1 9.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 3,215.0 3,139.7
Total assets 6,325.5 5,299.0
Total liabilities 4,658.1 3,664.8
Total White Mountains’s common shareholders’ equity 1,251.4 1,223.8
Noncontrolling interests 416.0 410.4
Financial Guarantee (HG Global) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 686.0 667.6
Total assets 1,203.1 1,179.4
Total liabilities 463.3 464.1
Total White Mountains’s common shareholders’ equity 752.3 728.7
Noncontrolling interests (12.5) (13.4)
Dividends payable 478.0 462.1
Asset Management (Kudu) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 1,138.1 1,041.9
Total assets 1,223.3 1,108.4
Total liabilities 330.6 316.7
Total White Mountains’s common shareholders’ equity 766.3 664.1
Noncontrolling interests 126.4 127.6
Bamboo | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 65.0 58.0
Total assets 615.0 584.6
Total liabilities 282.7 167.7
Total White Mountains’s common shareholders’ equity 241.2 303.3
Noncontrolling interests $ 91.1 $ 113.6