v3.26.1
Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Segment Reporting Information [Line Items]    
Earned insurance premiums $ 384.9 $ 395.0
Net investment income 71.4 59.6
Net realized and unrealized investment gains (losses) (54.4) 50.0
Interest income from BAM Surplus Notes 6.9 7.5
Commission and fee revenues 43.2 48.1
Net gain on sale of the Bamboo Group 2.4 0.0
Other revenues 63.4 17.6
Total revenues 517.8 577.8
Loss and LAE 207.0 261.8
Acquisition expenses 102.2 97.1
Cost of sales 42.7 7.5
Broker commission expenses 17.2 15.5
General and administrative expenses 144.9 96.0
Change in fair value of contingent consideration 10.0 9.7
Interest expense 19.3 17.8
Total expenses 543.3 505.4
Pre-tax income (loss) (25.5) 72.4
Other Operations    
Segment Reporting Information [Line Items]    
Earned insurance premiums 3.4 13.9
Net investment income 13.5 9.7
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 3.6 3.9
Net gain on sale of the Bamboo Group 2.4 0.0
Other revenues 56.4 13.6
Total revenues 21.0 7.3
Loss and LAE 0.3 17.4
Acquisition expenses 1.3 5.1
Cost of sales 42.7 7.5
Broker commission expenses 0.0 0.0
General and administrative expenses 55.3 35.5
Change in fair value of contingent consideration 0.0 0.0
Interest expense 1.0 0.5
Total expenses 100.6 66.0
Pre-tax income (loss) (79.6) (58.7)
Investments, Excluding MediaAlpha    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 10.8 86.6
Investments, Excluding MediaAlpha | Other Operations    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 6.9 2.8
MediaAlpha    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (65.2) (36.6)
MediaAlpha | Other Operations    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (65.2) (36.6)
P&C Insurance and Reinsurance (Ark/WM Outrigger)    
Segment Reporting Information [Line Items]    
Loss and LAE 206.7 233.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 373.8 358.0
Net investment income 28.7 23.5
Net realized and unrealized investment gains (losses) (32.9) 29.5
Other revenues 6.7 2.2
Loss and LAE 206.7 233.5
Acquisition expenses 98.8 83.5
General and administrative expenses 47.5 35.9
Change in fair value of contingent consideration 10.0 9.7
Interest expense 4.1 4.2
Total expenses 367.1 366.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 371.1 346.0
Net investment income 27.4 21.3
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 0.0 0.0
Net gain on sale of the Bamboo Group 0.0  
Other revenues 6.7 2.2
Total revenues 372.4 399.1
Loss and LAE 206.4 213.3
Acquisition expenses 97.9 83.8
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 47.5 35.8
Change in fair value of contingent consideration 10.0 9.7
Interest expense 4.1 4.2
Total expenses 365.9 346.8
Pre-tax income (loss) 6.5 52.3
Other underwriting expenses 34.9 28.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (32.8) 29.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0 0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 2.7 12.0
Net investment income 1.3 2.2
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 0.0 0.0
Net gain on sale of the Bamboo Group 0.0  
Other revenues 0.0 0.0
Total revenues 3.9 14.1
Loss and LAE 0.3 20.2
Acquisition expenses 0.9 (0.3)
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 0.0 0.1
Change in fair value of contingent consideration 0.0 0.0
Interest expense 0.0 0.0
Total expenses 1.2 20.0
Pre-tax income (loss) 2.7 (5.9)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (0.1) (0.1)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0 0.0
Asset Management (Kudu) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 0.0 0.0
Net investment income 20.8 19.4
Net realized and unrealized investment gains (losses) 42.0 44.0
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 0.0 0.0
Net gain on sale of the Bamboo Group 0.0  
Other revenues 0.2 0.4
Total revenues 63.0 63.8
Loss and LAE 0.0 0.0
Acquisition expenses 0.0 0.0
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 4.2 4.0
Change in fair value of contingent consideration 0.0 0.0
Interest expense 7.1 6.4
Total expenses 11.3 10.4
Pre-tax income (loss) 51.7 53.4
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 42.0 44.0
Asset Management (Kudu) | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0 0.0
Financial Guarantee (HG Global) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 7.7 8.2
Net investment income 7.7 6.3
Net realized and unrealized investment gains (losses) (5.2) 10.0
Interest income from BAM Surplus Notes 6.9 7.5
Other revenues 0.1 0.1
Acquisition expenses 2.1 1.9
General and administrative expenses 0.7 0.6
Interest expense 3.6 4.6
Total expenses 6.4 7.1
Financial Guarantee (HG Global) | HG Global | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 7.7 8.2
Net investment income 7.7 6.3
Interest income from BAM Surplus Notes 6.9 7.5
Commission and fee revenues 0.0 0.0
Net gain on sale of the Bamboo Group 0.0  
Other revenues 0.1 0.1
Total revenues 17.2 32.1
Loss and LAE 0.0 0.0
Acquisition expenses 2.1 1.9
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 0.7 0.6
Change in fair value of contingent consideration 0.0 0.0
Interest expense 3.6 4.6
Total expenses 6.4 7.1
Pre-tax income (loss) 10.8 25.0
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (5.2) 10.0
Financial Guarantee (HG Global) | HG Global | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0 0.0
P&C Insurance Distribution (Bamboo) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 0.0 14.9
Interest income from BAM Surplus Notes   0.0
Commission and fee revenues 0.0 44.2
Other revenues 0.0 2.3
Total revenues   61.4
Loss and LAE 0.0 10.9
Acquisition expenses 0.0 6.6
Cost of sales   0.0
Broker commission expenses 0.0 15.5
General and administrative expenses 0.0 20.0
Change in fair value of contingent consideration   0.0
Interest expense 0.0 2.1
Total expenses 0.0 55.1
Pre-tax income (loss)   6.3
Other intangible assets, finite-lived, accumulated amortization   4.0
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses)   0.0
P&C Insurance Distribution (Bamboo), net | Operating Segments    
Segment Reporting Information [Line Items]    
Net investment income   0.7
Other revenues   1.3
P&C Insurance Distribution (Bamboo), net | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses)   0.3
Distinguished | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 0.0  
Net investment income 0.7  
Interest income from BAM Surplus Notes 0.0  
Commission and fee revenues 39.6 0.0
Net gain on sale of the Bamboo Group 0.0  
Other revenues 0.7 0.0
Total revenues 40.3  
Loss and LAE 0.0  
Acquisition expenses 0.0  
Cost of sales 0.0  
Broker commission expenses 17.2 0.0
General and administrative expenses 37.2 0.0
Change in fair value of contingent consideration 0.0  
Interest expense 3.5 0.0
Total expenses 57.9 $ 0.0
Pre-tax income (loss) (17.6)  
Other intangible assets, finite-lived, accumulated amortization 7.4  
Distinguished | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0  
Distinguished | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0  
Distinguished, net | Operating Segments    
Segment Reporting Information [Line Items]    
Other revenues $ 0.0