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Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2024 | Dec. 31, 2023 | Dec. 31, 2022
Segment reporting information |  |  |
Earned insurance premiums | $ 1,691.6 | $ 1,440.9 | $ 1,076.7
Net investment income | 227.4 | 194.2 | 124.4
Net realized and unrealized investment gains (losses) | 184.9 | 434.2 | (191.5)
Interest income (expense) from BAM Surplus Notes | 15.8 | 0.0 | 0.0
Change in fair value of BAM Surplus Notes | 0.5 | 0.0 | 0.0
Unrealized loss on deconsolidation of BAM | (114.5) | 0.0 | 0.0
Commission and fees revenues | 149.4 | 13.2 | 11.5
Other revenues | 84.7 | 84.2 | 136.8
Total revenues | 2,239.8 | 2,166.7 | 1,157.9
Loss and loss adjustment expenses | 888.5 | 726.8 | 536.4
Acquisition expenses | 341.5 | 290.1 | 250.6
Cost of sales | 29.6 | 40.4 | 98.6
Broker commission expense | 51.3 |  |
General and administrative expenses | 490.1 | 432.6 | 364.1
Change in fair value of contingent consideration | 61.3 | 48.7 | 17.3
Interest expense | 60.8 | 62.7 | 40.3
Total expenses | 1,923.1 | 1,601.3 | 1,307.3
Pre-tax income (loss) from continuing operations | 316.7 | 565.4 | (149.4)
Other Operations |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 32.7 | 0.0 | 0.0
Net investment income | 35.6 | 30.1 | 32.2
Interest income (expense) from BAM Surplus Notes | 0.0 | 0.0 | 0.0
Change in fair value of BAM Surplus Notes | 0.0 |  |
Unrealized loss on deconsolidation of BAM | 0.0 |  |
Commission and fees revenues | 14.8 | 13.2 | 11.5
Other revenues | 56.8 | 80.5 | 127.2
Total revenues | 234.9 | 339.4 | 76.3
Loss and loss adjustment expenses | 12.1 | 0.0 | 0.0
Acquisition expenses | 12.1 | 0.0 | 0.0
Cost of sales | 29.6 | 40.4 | 98.6
Broker commission expense | 0.0 |  |
General and administrative expenses | 169.5 | 182.3 | 174.1
Change in fair value of contingent consideration | 0.0 | 0.0 | 0.0
Interest expense | 2.5 | 3.7 | 1.9
Total expenses | 225.8 | 226.4 | 274.6
Pre-tax income (loss) from continuing operations | 9.1 | 113.0 | (198.3)
Investments, Excluding MediaAlpha |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 146.9 | 407.1 | (98.5)
Investments, Excluding MediaAlpha | Other Operations |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 57.0 | 188.5 | (1.6)
MediaAlpha |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 38.0 | 27.1 | (93.0)
MediaAlpha | Other Operations |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 38.0 | 27.1 | (93.0)
P&C Insurance and Reinsurance (Ark/WM Outrigger) |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 1,587.8 | 1,409.7 | 1,043.4
Loss and loss adjustment expenses | 855.8 | 726.8 | 536.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 1,587.8 | 1,409.7 | 1,043.4
Net investment income | 90.7 | 61.4 | 16.3
Net realized and unrealized investment gains (losses) | 50.1 | 85.9 | (55.2)
Other revenues | 22.3 | 0.8 | 5.0
Loss and loss adjustment expenses | 855.8 | 726.8 | 536.4
Acquisition expenses | 307.1 | 281.5 | 239.4
General and administrative expenses | 208.4 | 162.0 | 106.2
Change in fair value of contingent consideration | 61.3 | 48.7 | 17.3
Interest expense | 19.5 | 21.3 | 15.1
Total expenses | 1,452.1 | 1,240.3 | 914.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 1,499.8 | 1,305.4 | 1,043.4
Net investment income | 79.4 | 50.4 | 16.3
Interest income (expense) from BAM Surplus Notes | 0.0 | 0.0 | 0.0
Change in fair value of BAM Surplus Notes | 0.0 |  |
Unrealized loss on deconsolidation of BAM | 0.0 |  |
Commission and fees revenues | 0.0 | 0.0 | 0.0
Other revenues | 22.3 | 0.8 | 5.0
Total revenues | 1,651.6 | 1,442.5 | 1,009.5
Loss and loss adjustment expenses | 825.9 | 711.2 | 536.4
Acquisition expenses | 283.9 | 251.0 | 239.4
Cost of sales | 0.0 | 0.0 | 0.0
Broker commission expense | 0.0 |  |
General and administrative expenses | 208.3 | 161.7 | 106.2
Change in fair value of contingent consideration | 61.3 | 48.7 | 17.3
Interest expense | 19.5 | 21.3 | 15.1
Total expenses | 1,398.9 | 1,193.9 | 914.4
Pre-tax income (loss) from continuing operations | 252.7 | 248.6 | 95.1
Other underwriting expenses | 136.1 | 113.6 | 78.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment | Investments, Excluding MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 50.1 | 85.9 | (55.2)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark Insurance Holdings Limited - Segment | MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 | 0.0 | 0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 88.0 | 104.3 |
Net investment income | 11.3 | 11.0 |
Interest income (expense) from BAM Surplus Notes | 0.0 | 0.0 |
Change in fair value of BAM Surplus Notes | 0.0 |  |
Unrealized loss on deconsolidation of BAM | 0.0 |  |
Commission and fees revenues | 0.0 | 0.0 |
Other revenues | 0.0 | 0.0 |
Total revenues | 99.3 | 115.3 |
Loss and loss adjustment expenses | 29.9 | 15.6 |
Acquisition expenses | 23.2 | 30.5 |
Cost of sales | 0.0 | 0.0 |
Broker commission expense | 0.0 |  |
General and administrative expenses | 0.1 | 0.3 |
Change in fair value of contingent consideration | 0.0 | 0.0 |
Interest expense | 0.0 | 0.0 |
Total expenses | 53.2 | 46.4 |
Pre-tax income (loss) from continuing operations | 46.1 | 68.9 |
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | Investments, Excluding MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 | 0.0 |
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 | 0.0 |
Financial Guarantee (HG Global) |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 31.7 | 31.2 | 33.3
Financial Guarantee (HG Global) | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 31.7 | 31.2 | 33.3
Net investment income | 32.2 | 31.7 | 21.5
Net realized and unrealized investment gains (losses) | (11.5) | 26.6 | (105.8)
Interest income (expense) from BAM Surplus Notes | 15.8 | 0.0 | 0.0
Change in fair value of BAM Surplus Notes | 0.5 | 0.0 | 0.0
Unrealized loss on deconsolidation of BAM | (114.5) | 0.0 | 0.0
Other revenues | 1.2 | 2.9 | 4.6
Acquisition expenses | 8.2 | 8.6 | 11.2
Broker commission expense | 0.0 |  |
General and administrative expenses | 35.7 | 68.9 | 69.1
Interest expense | 16.7 | 16.5 | 8.3
Total expenses | 60.6 | 94.0 | 88.6
Financial Guarantee (HG Global) | HG Global | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 28.9 | 26.0 | 27.5
Net investment income | 23.4 | 17.1 | 10.3
Interest income (expense) from BAM Surplus Notes | 29.0 | 26.2 | 11.7
Change in fair value of BAM Surplus Notes | 0.5 |  |
Unrealized loss on deconsolidation of BAM | (114.5) |  |
Commission and fees revenues | 0.0 | 0.0 | 0.0
Other revenues | 0.1 | 0.0 | 0.5
Total revenues | (39.0) | 82.9 | (2.5)
Loss and loss adjustment expenses | 0.0 | 0.0 | 0.0
Acquisition expenses | 7.8 | 7.4 | 9.3
Cost of sales | 0.0 | 0.0 | 0.0
Broker commission expense | 0.0 |  |
General and administrative expenses | 2.2 | 2.8 | 2.8
Change in fair value of contingent consideration | 0.0 | 0.0 | 0.0
Interest expense | 16.7 | 16.5 | 8.3
Total expenses | 26.7 | 26.7 | 20.4
Pre-tax income (loss) from continuing operations | (65.7) | 56.2 | (22.9)
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | (6.4) | 13.6 | (52.5)
Financial Guarantee (HG Global) | HG Global | MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 | 0.0 | 0.0
Financial Guarantee (HG Global) | BAM | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 2.8 | 5.2 | 5.8
Net investment income | 8.8 | 14.6 | 11.2
Interest income (expense) from BAM Surplus Notes | (13.2) | (26.2) | (11.7)
Change in fair value of BAM Surplus Notes | 0.0 |  |
Unrealized loss on deconsolidation of BAM | 0.0 |  |
Commission and fees revenues | 0.0 | 0.0 | 0.0
Other revenues | 1.1 | 2.9 | 4.1
Total revenues | (5.6) | 9.5 | (43.9)
Loss and loss adjustment expenses | 0.0 | 0.0 | 0.0
Acquisition expenses | 0.4 | 1.2 | 1.9
Cost of sales | 0.0 | 0.0 | 0.0
General and administrative expenses | 33.5 | 66.1 | 66.3
Change in fair value of contingent consideration | 0.0 | 0.0 | 0.0
Interest expense | 0.0 | 0.0 | 0.0
Total expenses | 33.9 | 67.3 | 68.2
Pre-tax income (loss) from continuing operations | (39.5) | (57.8) | (112.1)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | (5.1) | 13.0 | (53.3)
Financial Guarantee (HG Global) | BAM | MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 | 0.0 | 0.0
Asset Management (Kudu) | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 0.0 | 0.0 | 0.0
Net investment income | 66.7 | 71.0 | 54.4
Net realized and unrealized investment gains (losses) | 51.3 | 106.1 | 64.1
Interest income (expense) from BAM Surplus Notes | 0.0 | 0.0 | 0.0
Change in fair value of BAM Surplus Notes | 0.0 |  |
Unrealized loss on deconsolidation of BAM | 0.0 |  |
Commission and fees revenues | 0.0 | 0.0 | 0.0
Other revenues | 0.8 | 0.0 | 0.0
Total revenues | 118.8 | 177.1 | 118.5
Loss and loss adjustment expenses | 0.0 | 0.0 | 0.0
Acquisition expenses | 0.0 | 0.0 | 0.0
Cost of sales | 0.0 | 0.0 | 0.0
Broker commission expense | 0.0 |  |
General and administrative expenses | 15.4 | 19.4 | 14.7
Change in fair value of contingent consideration | 0.0 | 0.0 | 0.0
Interest expense | 22.1 | 21.2 | 15.0
Total expenses | 37.5 | 40.6 | 29.7
Pre-tax income (loss) from continuing operations | 81.3 | 136.5 | 88.8
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 51.3 | 106.1 | 64.1
Asset Management (Kudu) | MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 | 0.0 | 0.0
P&C Insurance Distribution (Bamboo) |  |  |
Segment reporting information |  |  |
Loss and loss adjustment expenses | 20.6 |  |
P&C Insurance Distribution (Bamboo) | Operating Segments |  |  |
Segment reporting information |  |  |
Earned insurance premiums | 39.4 | 0.0 | 0.0
Net investment income | 2.2 |  |
Interest income (expense) from BAM Surplus Notes | 0.0 |  |
Change in fair value of BAM Surplus Notes | 0.0 |  |
Unrealized loss on deconsolidation of BAM | 0.0 |  |
Commission and fees revenues | 134.6 | 0.0 | 0.0
Other revenues | 5.8 | 0.0 | 0.0
Total revenues | 179.8 |  |
Loss and loss adjustment expenses | 20.6 | 0.0 | 0.0
Acquisition expenses | 14.1 | 0.0 | 0.0
Cost of sales | 0.0 |  |
Broker commission expense | 51.3 | 0.0 | 0.0
General and administrative expenses | 61.1 | 0.0 | 0.0
Change in fair value of contingent consideration | 0.0 |  |
Interest expense | 0.0 |  |
Total expenses | 147.1 | $ 0.0 | $ 0.0
Pre-tax income (loss) from continuing operations | 32.7 |  |
Amortization of other intangibles | 16.4 |  |
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 |  |
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments |  |  |
Segment reporting information |  |  |
Net realized and unrealized investment gains (losses) | 0.0 |  |
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments |  |  |
Segment reporting information |  |  |
Other revenues | $ 3.6 |  |

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