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Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2024 | Mar. 31, 2023
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | $ 319.0 | $ 262.8
Net investment income | 57.0 | 39.0
Net investment income (expense) - BAM Surplus Note interest | 0.0 | 0.0
Net realized and unrealized investment gains (losses) | 226.8 | 198.1
Commission and fee revenues | 25.5 | 3.3
Other revenues | 19.0 | 28.7
Total revenues | 647.3 | 531.9
Loss and loss adjustment expenses | 185.8 | 147.8
Acquisition expenses | 71.6 | 62.5
Cost of sales | 7.6 | 13.9
Broker commission expenses | 9.3 |
General and administrative expenses | 125.2 | 96.0
Change in fair value of contingent consideration | 0.0 | (2.4)
Interest expense | 15.2 | 15.0
Total expenses | 414.7 | 332.8
Pre-tax income (loss) | 232.6 | 199.1
Other Operations |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 0.0 | 0.0
Net investment income | 9.9 | 7.0
Net investment income (expense) - BAM Surplus Note interest | 0.0 | 0.0
Commission and fee revenues | 3.6 | 3.3
Other revenues | 14.4 | 30.6
Total revenues | 260.8 | 167.9
Loss and loss adjustment expenses | 0.0 | 0.0
Acquisition expenses | 0.0 | 0.0
Cost of sales | 7.6 | 13.9
Broker commission expenses | 0.0 |
General and administrative expenses | 50.3 | 39.7
Change in fair value of contingent consideration | 0.0 | 0.0
Interest expense | 0.7 | 0.8
Total expenses | 58.6 | 54.4
Pre-tax income (loss) | 202.2 | 113.5
Investments, Excluding MediaAlpha |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | 16.1 | 112.9
Investments, Excluding MediaAlpha | Other Operations |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | 22.2 | 41.8
MediaAlpha |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | 210.7 | 85.2
MediaAlpha | Other Operations |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | 210.7 | 85.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |
Segment Reporting Information [Line Items] |  |
Loss and loss adjustment expenses | 180.0 | 147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 302.8 | 255.1
Net investment income | 19.9 | 10.6
Net realized and unrealized investment gains (losses) | 10.6 | 24.5
Other revenues | 3.5 | (2.7)
Total revenues | 336.8 | 287.5
Loss and loss adjustment expenses | 180.0 | 147.8
Acquisition expenses | 66.3 | 59.8
General and administrative expenses | 42.2 | 35.2
Change in fair value of contingent consideration | 0.0 | (2.4)
Interest expense | 5.4 | 5.0
Total expenses | 293.9 | 245.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | UNITED KINGDOM |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 177.4 | 146.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | BERMUDA |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 115.1 | 104.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 292.5 | 249.9
Net investment income | 17.0 | 8.4
Net investment income (expense) - BAM Surplus Note interest | 0.0 | 0.0
Commission and fee revenues | 0.0 | 0.0
Other revenues | 3.5 | (2.7)
Total revenues | 323.6 | 280.1
Loss and loss adjustment expenses | 179.3 | 147.6
Acquisition expenses | 63.7 | 58.9
Cost of sales | 0.0 | 0.0
Broker commission expenses | 0.0 |
General and administrative expenses | 42.2 | 35.1
Change in fair value of contingent consideration | 0.0 | (2.4)
Interest expense | 5.4 | 5.0
Total expenses | 290.6 | 244.2
Pre-tax income (loss) | 33.0 | 35.9
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) | 10.6 | 24.5
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 | 10.3 | 5.2
Net investment income | 2.9 | 2.2
Net investment income (expense) - BAM Surplus Note interest | 0.0 | 0.0
Commission and fee revenues | 0.0 | 0.0
Other revenues | 0.0 | 0.0
Total revenues | 13.2 | 7.4
Loss and loss adjustment expenses | 0.7 | 0.2
Acquisition expenses | 2.6 | 0.9
Cost of sales | 0.0 | 0.0
Broker commission expenses | 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 | 3.3 | 1.2
Pre-tax income (loss) | 9.9 | 6.2
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) | 0.0 | 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/BAM) | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 7.8 | 7.7
Net investment income | 9.7 | 7.2
Net realized and unrealized investment gains (losses) | (10.1) | 17.0
Other revenues | 0.5 | 0.8
Total revenues | 7.9 | 32.7
Acquisition expenses | 2.2 | 2.7
General and administrative expenses | 17.3 | 17.3
Interest expense | 3.5 | 4.5
Total expenses | 23.0 | 24.5
Financial Guarantee (HG Global/BAM) | HG Global | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 6.5 | 6.4
Net investment income | 5.4 | 4.0
Net investment income (expense) - BAM Surplus Note interest | 6.6 | 6.6
Commission and fee revenues | 0.0 | 0.0
Other revenues | 0.0 | 0.0
Total revenues | 11.2 | 24.9
Loss and loss adjustment expenses | 0.0 | 0.0
Acquisition expenses | 1.8 | 1.8
Cost of sales | 0.0 | 0.0
Broker commission expenses | 0.0 |
General and administrative expenses | 0.4 | 1.1
Change in fair value of contingent consideration | 0.0 | 0.0
Interest expense | 3.5 | 4.5
Total expenses | 5.7 | 7.4
Pre-tax income (loss) | 5.5 | 17.5
Financial Guarantee (HG Global/BAM) | HG Global | Investments, Excluding MediaAlpha | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | (7.3) | 7.9
Financial Guarantee (HG Global/BAM) | 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) | BAM | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Earned insurance premiums | 1.3 | 1.3
Net investment income | 4.3 | 3.2
Net investment income (expense) - BAM Surplus Note interest | (6.6) | (6.6)
Commission and fee revenues | 0.0 | 0.0
Other revenues | 0.5 | 0.8
Total revenues | (3.3) | 7.8
Loss and loss adjustment expenses | 0.0 | 0.0
Acquisition expenses | 0.4 | 0.9
Cost of sales | 0.0 | 0.0
Broker commission expenses | 0.0 |
General and administrative expenses | 16.9 | 16.2
Change in fair value of contingent consideration | 0.0 | 0.0
Interest expense | 0.0 | 0.0
Total expenses | 17.3 | 17.1
Pre-tax income (loss) | (20.6) | (9.3)
Financial Guarantee (HG Global/BAM) | BAM | Investments, Excluding MediaAlpha | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | (2.8) | 9.1
Financial Guarantee (HG Global/BAM) | BAM | 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 | 17.2 | 14.2
Net investment income (expense) - BAM Surplus Note interest | 0.0 | 0.0
Net realized and unrealized investment gains (losses) | (6.5) | 29.6
Commission and fee revenues | 0.0 | 0.0
Other revenues | 0.0 | 0.0
Total revenues | 10.7 | 43.8
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 |
General and administrative expenses | 3.4 | 3.8
Change in fair value of contingent consideration | 0.0 | 0.0
Interest expense | 5.6 | 4.7
Total expenses | 9.0 | 8.5
Pre-tax income (loss) | 1.7 | 35.3
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | (6.5) | 29.6
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 | 8.4 | 0.0
Net investment income | 0.3 |
Net investment income (expense) - BAM Surplus Note interest | 0.0 |
Commission and fee revenues | 21.9 | 0.0
Other revenues | 0.8 | 0.0
Total revenues | 31.1 | 0.0
Loss and loss adjustment expenses | 5.8 | 0.0
Acquisition expenses | 3.1 | 0.0
Cost of sales | 0.0 |
Broker commission expenses | 9.3 | 0.0
General and administrative expenses | 12.0 | 0.0
Change in fair value of contingent consideration | 0.0 |
Interest expense | 0.0 |
Total expenses | 30.2 | $ 0.0
Pre-tax income (loss) | 0.9 |
Bamboo | Investments, Excluding MediaAlpha | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | (0.1) |
Bamboo | MediaAlpha | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Net realized and unrealized investment gains (losses) | 0.0 |
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments |  |
Segment Reporting Information [Line Items] |  |
Other revenues | $ 0.6 |

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