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Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	12 Months Ended
	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

X

- Definition

The total amount of expense recognized during the period for future policy benefits, claims and claims adjustment costs, and for selling, general and administrative costs.

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