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Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	11 Months Ended	12 Months Ended
	Dec. 05, 2025	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Segment reporting information
Earned insurance premiums		$ 1,777.1	$ 1,691.6	$ 1,440.9
Net investment income		248.0	227.4	194.2
Net realized and unrealized investment gains (losses)		361.5	184.9	434.2
Interest income from BAM Surplus Notes		29.8	15.8	0.0
Change in fair value of BAM Surplus Notes		(37.5)	0.5	0.0
Unrealized loss on deconsolidation of BAM		0.0	(114.5)	0.0
Commission and fee revenues		284.5	149.4	13.2
Net gain on sale of the Bamboo Group		849.3	0.0	0.0
Other revenues		222.3	84.7	84.2
Total revenues		3,735.0	2,239.8	2,166.7
Loss and loss adjustment expenses		882.2	888.5	726.8
Acquisition expenses		420.2	341.5	290.1
Cost of sales		151.8	29.6	40.4
Broker commission expense		94.1	51.3
General and administrative expenses		606.6	490.1	432.6
Change in fair value of contingent consideration		173.0	61.3	48.7
Interest expense		78.4	60.8	62.7
Total expenses		2,406.3	1,923.1	1,601.3
Pre-tax income (loss)		1,328.7	316.7	565.4
Other intangible assets, finite-lived, accumulated amortization		38.9	39.0
Other Operations
Segment reporting information
Earned insurance premiums		22.2	32.7	0.0
Net investment income		32.1	35.6	30.1
Interest income 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 fee revenues		16.4	14.8	13.2
Net gain on sale of the Bamboo Group		849.3	0.0	0.0
Other revenues		200.6	56.8	80.5
Total revenues		1,230.2	234.9	339.4
Loss and loss adjustment expenses		20.0	12.1	0.0
Acquisition expenses		8.3	12.1	0.0
Cost of sales		151.8	29.6	40.4
Broker commission expense		0.0	0.0
General and administrative expenses		236.7	169.5	182.3
Change in fair value of contingent consideration		0.0	0.0	0.0
Interest expense		3.2	2.5	3.7
Total expenses		420.0	225.8	226.4
Pre-tax income (loss)		810.2	9.1	113.0
Other intangible assets, finite-lived, accumulated amortization		29.0	20.8
Investments, Excluding MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)		331.9	146.9	407.1
Investments, Excluding MediaAlpha | Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)		80.0	57.0	188.5
MediaAlpha
Segment reporting information
Net realized and unrealized investment gains (losses)		29.6	38.0	27.1
Change in fair value of BAM Surplus Notes			0.5
MediaAlpha | Other Operations
Segment reporting information
Net realized and unrealized investment gains (losses)		29.6	38.0	27.1
Change in fair value of BAM Surplus Notes			0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment reporting information
Earned insurance premiums		1,697.4	1,587.8	1,409.7
Loss and loss adjustment expenses		844.2	855.8	726.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment reporting information
Earned insurance premiums		1,697.4	1,587.8	1,409.7
Net investment income		106.5	90.7	61.4
Net realized and unrealized investment gains (losses)		124.6	50.1	85.9
Other revenues		15.3	22.3	0.8
Loss and loss adjustment expenses		844.2	855.8	726.8
Acquisition expenses		394.2	307.1	281.5
General and administrative expenses		205.1	208.4	162.0
Change in fair value of contingent consideration		173.0	61.3	48.7
Interest expense		17.1	19.5	21.3
Total expenses		1,633.6	1,452.1	1,240.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment reporting information
Earned insurance premiums		1,612.8	1,499.8	1,305.4
Net investment income		97.4	79.4	50.4
Interest income 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 fee revenues		0.0	0.0	0.0
Net gain on sale of the Bamboo Group		0.0
Other revenues		15.3	22.3	0.8
Total revenues		1,850.1	1,651.6	1,442.5
Loss and loss adjustment expenses		818.8	825.9	711.2
Acquisition expenses		371.1	283.9	251.0
Cost of sales		0.0	0.0	0.0
Broker commission expense		0.0	0.0
General and administrative expenses		205.0	208.3	161.7
Change in fair value of contingent consideration		173.0	61.3	48.7
Interest expense		17.1	19.5	21.3
Total expenses		1,585.0	1,398.9	1,193.9
Pre-tax income (loss)		265.1	252.7	248.6
Other underwriting expenses		143.9	136.1	113.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		124.6	50.1	85.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		0.0	0.0	0.0
Change in fair value of BAM Surplus Notes			0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re - Segment | Operating Segments
Segment reporting information
Earned insurance premiums		84.6	88.0	104.3
Net investment income		9.1	11.3	11.0
Interest income 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 fee revenues		0.0	0.0	0.0
Net gain on sale of the Bamboo Group		0.0
Other revenues		0.0	0.0	0.0
Total revenues		93.7	99.3	115.3
Loss and loss adjustment expenses		25.4	29.9	15.6
Acquisition expenses		23.1	23.2	30.5
Cost of sales		0.0	0.0	0.0
Broker commission expense		0.0	0.0
General and administrative expenses		0.1	0.1	0.3
Change in fair value of contingent consideration		0.0	0.0	0.0
Interest expense		0.0	0.0	0.0
Total expenses		48.6	53.2	46.4
Pre-tax income (loss)		45.1	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	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	0.0
Change in fair value of BAM Surplus Notes			0.0
Financial Guarantee (HG Global)
Segment reporting information
Earned insurance premiums		30.8	31.7	31.2
Financial Guarantee (HG Global) | Operating Segments
Segment reporting information
Earned insurance premiums		30.8	31.7	31.2
Net investment income		27.1	32.2	31.7
Net realized and unrealized investment gains (losses)		23.2	(11.5)	26.6
Interest income from BAM Surplus Notes		29.8	15.8	0.0
Change in fair value of BAM Surplus Notes		(37.5)	0.5	0.0
Unrealized loss on deconsolidation of BAM		0.0	(114.5)	0.0
Other revenues		0.2	1.2	2.9
Acquisition expenses		8.0	8.2	8.6
General and administrative expenses		3.1	35.7	68.9
Interest expense		17.4	16.7	16.5
Total expenses		28.5	60.6	94.0
Financial Guarantee (HG Global) | HG Global | Operating Segments
Segment reporting information
Earned insurance premiums		30.8	28.9	26.0
Net investment income		27.1	23.4	17.1
Interest income from BAM Surplus Notes		29.8	29.0	26.2
Change in fair value of BAM Surplus Notes		(37.5)
Unrealized loss on deconsolidation of BAM			(114.5)
Commission and fee revenues		0.0	0.0	0.0
Net gain on sale of the Bamboo Group		0.0
Other revenues		0.2	0.1	0.0
Total revenues		73.6	(39.0)	82.9
Loss and loss adjustment expenses		0.0	0.0	0.0
Acquisition expenses		8.0	7.8	7.4
Cost of sales		0.0	0.0	0.0
Broker commission expense		0.0	0.0
General and administrative expenses		3.1	2.2	2.8
Change in fair value of contingent consideration		0.0	0.0	0.0
Interest expense		17.4	16.7	16.5
Total expenses		28.5	26.7	26.7
Pre-tax income (loss)		45.1	(65.7)	56.2
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		23.2	(6.4)	13.6
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
Change in fair value of BAM Surplus Notes			0.5
Financial Guarantee (HG Global) | BAM | Operating Segments
Segment reporting information
Earned insurance premiums			2.8	5.2
Net investment income			8.8	14.6
Interest income from BAM Surplus Notes			(13.2)	(26.2)
Unrealized loss on deconsolidation of BAM			0.0
Commission and fee revenues			0.0	0.0
Other revenues			1.1	2.9
Total revenues			(5.6)	9.5
Loss and loss adjustment expenses			0.0	0.0
Acquisition expenses			0.4	1.2
Cost of sales			0.0	0.0
Broker commission expense			0.0
General and administrative expenses			33.5	66.1
Change in fair value of contingent consideration			0.0	0.0
Interest expense			0.0	0.0
Total expenses			33.9	67.3
Pre-tax income (loss)			(39.5)	(57.8)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)			(5.1)	13.0
Financial Guarantee (HG Global) | BAM | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)			0.0	0.0
Change in fair value of BAM Surplus Notes			0.0
Asset Management (Kudu) | Operating Segments
Segment reporting information
Earned insurance premiums		0.0	0.0	0.0
Net investment income		78.7	66.7	71.0
Net realized and unrealized investment gains (losses)		103.5	51.3	106.1
Interest income 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 fee revenues		0.0	0.0	0.0
Net gain on sale of the Bamboo Group		0.0
Other revenues		1.2	0.8	0.0
Total revenues		183.4	118.8	177.1
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	0.0
General and administrative expenses		17.9	15.4	19.4
Change in fair value of contingent consideration		0.0	0.0	0.0
Interest expense		25.9	22.1	21.2
Total expenses		43.8	37.5	40.6
Pre-tax income (loss)		139.6	81.3	136.5
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		103.5	51.3	106.1
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		0.0	0.0	0.0
Change in fair value of BAM Surplus Notes			0.0
P&C Insurance Distribution (Bamboo) | Operating Segments
Segment reporting information
Earned insurance premiums		26.7	39.4	0.0
Net investment income		2.6	2.2
Interest income 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 fee revenues		211.4	134.6	0.0
Net gain on sale of the Bamboo Group		0.0
Other revenues		5.0	3.6
Other revenues		8.2	5.8	0.0
Total revenues		246.3	179.8
Loss and loss adjustment expenses	$ 18.0	18.0	20.6	0.0
Acquisition expenses		9.7	14.1	0.0
Cost of sales		0.0	0.0
Broker commission expense		72.0	51.3	0.0
General and administrative expenses		96.8	61.1	0.0
Change in fair value of contingent consideration		0.0	0.0
Interest expense		9.7	0.0	0.0
Total expenses		206.2	147.1	0.0
Pre-tax income (loss)		40.1	32.7
Amortization of other intangibles		12.0	16.4
Other intangible assets, finite-lived, accumulated amortization		0.0	16.4
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		0.6	0.0
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		0.0	0.0
Change in fair value of BAM Surplus Notes			0.0
Distinguished Partners | Operating Segments
Segment reporting information
Earned insurance premiums		0.0
Net investment income		1.0
Interest income from BAM Surplus Notes		0.0
Change in fair value of BAM Surplus Notes		0.0
Commission and fee revenues		56.7	0.0	0.0
Net gain on sale of the Bamboo Group		0.0
Other revenues		0.0
Other revenues		1.0	0.0	0.0
Total revenues		57.7
Loss and loss adjustment expenses		0.0
Acquisition expenses		0.0
Cost of sales		0.0
Broker commission expense		22.1	0.0	0.0
General and administrative expenses		47.0	0.0	0.0
Change in fair value of contingent consideration		0.0
Interest expense		5.1	0.0	0.0
Total expenses		74.2	0.0	$ 0.0
Pre-tax income (loss)		(16.5)
Other intangible assets, finite-lived, accumulated amortization		7.8	$ 0.0
Distinguished Partners | Investments, Excluding MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		0.0
Distinguished Partners | MediaAlpha | Operating Segments
Segment reporting information
Net realized and unrealized investment gains (losses)		$ 0.0

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- Definition

General and Administrative Expense, Amortization of Intangible Assets

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- Definition

General and Administrative Expense, Other Underwriting Expenses

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- Definition

Other Income, Less Net Investment Income and Gain (Loss) on Investments

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- Definition

Revenues, Adjusted

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