v3.26.1
Segment Information - Assets (Details) - USD ($)
$ in Millions
Jun. 30, 2026
Dec. 31, 2025
Segment Reporting [Line Items]    
Total investments $ 8,134.4 $ 8,323.9
Total assets 13,766.7 12,306.5
Total liabilities 7,514.7 6,051.4
Redeemable noncontrolling interests 131.5 131.5
Total White Mountains’s common shareholders’ equity 5,388.3 5,425.4
Nonredeemable noncontrolling interests 732.2 698.2
Other Operations    
Segment Reporting [Line Items]    
Total investments 2,120.6 2,205.4
Total assets 2,303.3 2,395.5
Total liabilities 118.5 178.0
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 2,180.0 2,211.9
Nonredeemable noncontrolling interests 4.8 5.5
Other Operations | Other Disposal Group    
Segment Reporting [Line Items]    
Total investments 1.6 1.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting [Line Items]    
Total investments 3,725.5 3,926.5
Total assets 7,570.2 6,354.1
Total liabilities 5,864.0 4,528.5
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 1,234.7 1,360.4
Nonredeemable noncontrolling interests 471.5 465.2
Asset Management (Kudu) | Operating Segments    
Segment Reporting [Line Items]    
Total investments 1,459.0 1,313.3
Total assets 1,496.9 1,402.3
Total liabilities 467.4 446.9
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 885.4 814.2
Nonredeemable noncontrolling interests 144.1 141.2
HG Global | Operating Segments    
Segment Reporting [Line Items]    
Total investments 749.3 784.2
Total assets 1,216.6 1,236.8
Total liabilities 549.3 499.5
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 687.3 755.7
Nonredeemable noncontrolling interests (20.0) (18.4)
Dividends payable 470.7 527.2
Distinguished | Operating Segments    
Segment Reporting [Line Items]    
Total investments 79.4 94.0
Total assets 744.8 735.4
Total liabilities 332.2 307.1
Redeemable noncontrolling interests 131.5 131.5
Total White Mountains’s common shareholders’ equity 205.1 223.1
Nonredeemable noncontrolling interests 76.0 73.7
WTM Partners | Operating Segments    
Segment Reporting [Line Items]    
Total investments 0.6 0.5
Total assets 434.9 182.4
Total liabilities 183.3 91.4
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 195.8 60.1
Nonredeemable noncontrolling interests $ 55.8 $ 31.0