v3.26.3
Segment Information - Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2026
Dec. 31, 2025
Segment Reporting [Line Items]    
Total investments $ 8,280.9 $ 8,323.9
Total assets 13,362.6 12,306.5
Total liabilities 7,136.3 6,051.4
Redeemable noncontrolling interests 131.5 131.5
Total White Mountains’s common shareholders’ equity 5,353.1 5,425.4
Nonredeemable noncontrolling interests 741.7 698.2
Other Operations    
Segment Reporting [Line Items]    
Total investments 2,203.9 2,205.4
Total assets 2,369.0 2,395.5
Total liabilities 125.4 178.0
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 2,238.9 2,212.0
Nonredeemable noncontrolling interests 4.7 5.5
Other Operations | Other Disposal Group    
Segment Reporting [Line Items]    
Total investments 1.5 1.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting [Line Items]    
Total investments 3,817.5 3,926.5
Total assets 7,147.3 6,354.1
Total liabilities 5,459.8 4,528.5
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 1,197.6 1,360.4
Nonredeemable noncontrolling interests 489.9 465.2
Asset Management (Kudu) | Operating Segments    
Segment Reporting [Line Items]    
Total investments 1,440.0 1,313.3
Total assets 1,478.2 1,402.3
Total liabilities 473.5 446.9
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 863.9 814.2
Nonredeemable noncontrolling interests 140.8 141.2
HG Global | Operating Segments    
Segment Reporting [Line Items]    
Total investments 748.3 784.2
Total assets 1,205.7 1,236.8
Total liabilities 566.6 499.5
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 664.2 755.7
Nonredeemable noncontrolling interests (25.1) (18.4)
Dividends payable 486.8 527.2
Distinguished | Operating Segments    
Segment Reporting [Line Items]    
Total investments 70.6 94.0
Total assets 720.9 735.4
Total liabilities 317.4 307.1
Redeemable noncontrolling interests 131.5 131.5
Total White Mountains’s common shareholders’ equity 196.0 223.1
Nonredeemable noncontrolling interests 76.0 73.7
WTM Partners | Operating Segments    
Segment Reporting [Line Items]    
Total investments 0.6 0.5
Total assets 441.5 182.4
Total liabilities 193.6 91.4
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 192.5 60.0
Nonredeemable noncontrolling interests $ 55.4 $ 31.0