v3.25.1
Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2025
Mar. 31, 2024
Segment Reporting Information [Line Items]    
Earned insurance premiums $ 395.0 $ 319.0
Net investment income 59.6 57.0
Net realized and unrealized investment gains (losses) 50.0 226.8
Interest income from BAM Surplus Notes 7.5 0.0
Commission and fee revenues 48.1 25.5
Other revenues 17.6 19.0
Total revenues 577.8 647.3
Loss and loss adjustment expenses 261.8 185.8
Acquisition expenses 97.1 71.6
Cost of sales 7.5 7.6
Broker commission expenses 15.5 9.3
General and administrative expenses 96.0 125.2
Change in fair value of contingent consideration 9.7 0.0
Interest expense 17.8 15.2
Total expenses 505.4 414.7
Pre-tax income (loss) 72.4 232.6
UNITED KINGDOM    
Segment Reporting Information [Line Items]    
Earned insurance premiums 215.5 177.4
Commission and fee revenues 0.0 0.0
BERMUDA    
Segment Reporting Information [Line Items]    
Earned insurance premiums 164.6 140.3
Commission and fee revenues 0.0 0.0
Other Operations    
Segment Reporting Information [Line Items]    
Earned insurance premiums 13.9 0.0
Net investment income 9.7 9.9
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 3.9 3.6
Other revenues 13.6 14.4
Total revenues 7.3 260.8
Loss and loss adjustment expenses 17.4 0.0
Acquisition expenses 5.1 0.0
Cost of sales 7.5 7.6
Broker commission expenses 0.0 0.0
General and administrative expenses 35.5 50.3
Change in fair value of contingent consideration 0.0 0.0
Interest expense 0.5 0.7
Total expenses 66.0 58.6
Pre-tax income (loss) (58.7) 202.2
Investments, Excluding MediaAlpha    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 86.6 16.1
Investments, Excluding MediaAlpha | Other Operations    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 2.8 22.2
MediaAlpha    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (36.6) 210.7
MediaAlpha | Other Operations    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) (36.6) 210.7
P&C Insurance and Reinsurance (Ark/WM Outrigger)    
Segment Reporting Information [Line Items]    
Loss and loss adjustment expenses 233.5 180.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 358.0 302.8
Net investment income 23.5 19.9
Net realized and unrealized investment gains (losses) 29.5 10.6
Other revenues 2.2 3.5
Loss and loss adjustment expenses 233.5 180.0
Acquisition expenses 83.5 66.3
General and administrative expenses 35.9 42.2
Change in fair value of contingent consideration 9.7 0.0
Interest expense 4.2 5.4
Total expenses 366.8 293.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 346.0 292.5
Net investment income 21.3 17.0
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 0.0 0.0
Other revenues 2.2 3.5
Total revenues 399.1 323.6
Loss and loss adjustment expenses 213.3 179.3
Acquisition expenses 83.8 63.7
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 35.8 42.2
Change in fair value of contingent consideration 9.7 0.0
Interest expense 4.2 5.4
Total expenses 346.8 290.6
Pre-tax income (loss) 52.3 33.0
Other underwriting expenses 28.5 30.5
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) 29.6 10.6
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 12.0 10.3
Net investment income 2.2 2.9
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 0.0 0.0
Other revenues 0.0 0.0
Total revenues 14.1 13.2
Loss and loss adjustment expenses 20.2 0.7
Acquisition expenses (0.3) 2.6
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 0.1 0.0
Change in fair value of contingent consideration 0.0 0.0
Interest expense 0.0 0.0
Total expenses 20.0 3.3
Pre-tax income (loss) (5.9) 9.9
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) 0.0
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
Financial Guarantee (HG Global) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 8.2 7.8
Net investment income 6.3 9.7
Net realized and unrealized investment gains (losses) 10.0 (10.1)
Interest income from BAM Surplus Notes 7.5 0.0
Other revenues 0.1 0.5
Acquisition expenses 1.9 2.2
Broker commission expenses   0.0
General and administrative expenses 0.6 17.3
Interest expense 4.6 3.5
Total expenses 7.1 23.0
Financial Guarantee (HG Global) | HG Global | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 8.2 6.5
Net investment income 6.3 5.4
Interest income from BAM Surplus Notes 7.5 6.6
Commission and fee revenues 0.0 0.0
Other revenues 0.1 0.0
Total revenues 32.1 11.2
Loss and loss adjustment expenses 0.0 0.0
Acquisition expenses 1.9 1.8
Cost of sales 0.0 0.0
Broker commission expenses 0.0 0.0
General and administrative expenses 0.6 0.4
Change in fair value of contingent consideration 0.0 0.0
Interest expense 4.6 3.5
Total expenses 7.1 5.7
Pre-tax income (loss) 25.0 5.5
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 10.0 (7.3)
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
Financial Guarantee (HG Global) | BAM | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums   1.3
Net investment income   4.3
Interest income from BAM Surplus Notes   (6.6)
Commission and fee revenues   0.0
Other revenues   0.5
Total revenues   (3.3)
Loss and loss adjustment expenses   0.0
Acquisition expenses   0.4
Cost of sales   0.0
General and administrative expenses   16.9
Change in fair value of contingent consideration   0.0
Interest expense   0.0
Total expenses   17.3
Pre-tax income (loss)   (20.6)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses)   (2.8)
Financial Guarantee (HG Global) | BAM | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses)   0.0
Asset Management (Kudu) | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 0.0 0.0
Net investment income 19.4 17.2
Net realized and unrealized investment gains (losses) 44.0 (6.5)
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 0.0 0.0
Other revenues 0.4 0.0
Total revenues 63.8 10.7
Loss and loss adjustment expenses 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 4.0 3.4
Change in fair value of contingent consideration 0.0 0.0
Interest expense 6.4 5.6
Total expenses 10.4 9.0
Pre-tax income (loss) 53.4 1.7
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 44.0 (6.5)
Asset Management (Kudu) | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0 0.0
Bamboo | Operating Segments    
Segment Reporting Information [Line Items]    
Earned insurance premiums 14.9 8.4
Net investment income 0.7 0.3
Interest income from BAM Surplus Notes 0.0 0.0
Commission and fee revenues 44.2 21.9
Total revenues 61.4 31.1
Loss and loss adjustment expenses 10.9 5.8
Acquisition expenses 6.6 3.1
Cost of sales 0.0 0.0
Broker commission expenses 15.5 9.3
General and administrative expenses 20.0 12.0
Change in fair value of contingent consideration 0.0 0.0
Interest expense 2.1 0.0
Total expenses 55.1 30.2
Pre-tax income (loss) 6.3 0.9
Amortization of other intangibles 4.0 4.2
Bamboo | Investments, Excluding MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.3 (0.1)
Bamboo | MediaAlpha | Operating Segments    
Segment Reporting Information [Line Items]    
Net realized and unrealized investment gains (losses) 0.0 0.0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments    
Segment Reporting Information [Line Items]    
Other revenues $ 1.3 $ 0.6