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Segment Information - Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2025
Dec. 31, 2024
Segment reporting information    
Total investments $ 8,323.9 $ 6,417.6
Total assets 12,306.5 9,925.6
Total liabilities 6,051.4 4,794.6
Redeemable noncontrolling interests 131.5 0.0
Total White Mountains’s common shareholders’ equity 5,425.4 4,483.7
Nonredeemable noncontrolling interests 698.2 647.3
Operating Segments | P&C Insurance and Reinsurance (Ark/WM Outrigger)    
Segment reporting information    
Total investments 3,926.5 3,139.7
Total assets 6,354.1 5,299.0
Total liabilities 4,528.5 3,664.8
Redeemable noncontrolling interests 0.0  
Total White Mountains’s common shareholders’ equity 1,360.4 1,223.8
Nonredeemable noncontrolling interests 465.2 410.4
Operating Segments | Financial Guarantee (HG Global)    
Segment reporting information    
Total investments 784.2 667.6
Total assets 1,236.8 1,179.4
Total liabilities 499.5 464.1
Redeemable noncontrolling interests 0.0  
Total White Mountains’s common shareholders’ equity 755.7 728.7
Nonredeemable noncontrolling interests (18.4) (13.4)
Dividends payable 527.2 462.1
Operating Segments | Asset Management (Kudu)    
Segment reporting information    
Total investments 1,313.3 1,041.9
Total assets 1,402.3 1,108.4
Total liabilities 446.9 316.7
Redeemable noncontrolling interests 0.0  
Total White Mountains’s common shareholders’ equity 814.2 664.1
Nonredeemable noncontrolling interests 141.2 127.6
Operating Segments | Specialty Insurance Distribution (Distinguished)    
Segment reporting information    
Total investments 94.0 0.0
Total assets 735.4 0.0
Total liabilities 307.1 0.0
Redeemable noncontrolling interests 131.5  
Total White Mountains’s common shareholders’ equity 223.1 0.0
Nonredeemable noncontrolling interests 73.7 0.0
Other Operations    
Segment reporting information    
Total investments 2,205.9 1,568.4
Total assets 2,577.9 2,338.8
Total liabilities 269.4 349.0
Redeemable noncontrolling interests 0.0  
Total White Mountains’s common shareholders’ equity 2,272.0 1,867.1
Nonredeemable noncontrolling interests 36.5 122.7
Other Operations | Other Disposal Group    
Segment reporting information    
Total investments $ 1.0 2.0
Other Operations | Bamboo Group    
Segment reporting information    
Total investments   $ 58.0