v3.26.1
Segment Information - Assets (Details) - USD ($)
$ in Millions
Mar. 31, 2026
Dec. 31, 2025
Segment Reporting Information [Line Items]    
Total investments $ 8,353.4 $ 8,323.9
Total assets 13,168.9 12,306.5
Total liabilities 6,992.9 6,051.4
Redeemable noncontrolling interests 131.5 131.5
Total White Mountains’s common shareholders’ equity 5,373.5 5,425.4
Nonredeemable noncontrolling interests 671.0 698.2
Other Operations    
Segment Reporting Information [Line Items]    
Total investments 2,209.4 2,205.9
Total assets 2,552.4 2,577.9
Total liabilities 211.1 269.4
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 2,304.9 2,272.0
Nonredeemable noncontrolling interests 36.4 36.5
Other Operations | Other Disposal Group    
Segment Reporting Information [Line Items]    
Total investments 0.6 1.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 3,907.4 3,926.5
Total assets 7,220.7 6,354.1
Total liabilities 5,541.4 4,528.5
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 1,231.5 1,360.4
Nonredeemable noncontrolling interests 447.8 465.2
Asset Management (Kudu) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 1,379.7 1,313.3
Total assets 1,447.1 1,402.3
Total liabilities 456.0 446.9
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 855.7 814.2
Nonredeemable noncontrolling interests 135.4 141.2
Financial Guarantee (HG Global) | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 790.8 784.2
Total assets 1,247.6 1,236.8
Total liabilities 499.8 499.5
Redeemable noncontrolling interests 0.0 0.0
Total White Mountains’s common shareholders’ equity 767.0 755.7
Nonredeemable noncontrolling interests (19.2) (18.4)
Dividends payable 544.1 527.2
Distinguished | Operating Segments    
Segment Reporting Information [Line Items]    
Total investments 66.1 94.0
Total assets 701.1 735.4
Total liabilities 284.6 307.1
Redeemable noncontrolling interests 131.5 131.5
Total White Mountains’s common shareholders’ equity 214.4 223.1
Nonredeemable noncontrolling interests $ 70.6 $ 73.7