Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 557.8	$ 581.5	$ 1,328.0	$ 1,244.4
Net investment income	59.2	57.7	180.2	172.1
Net realized and unrealized investment gains (losses)	83.9	223.8	251.2	390.4
Interest income from BAM Surplus Notes	7.5	7.9	22.5	7.9
Change in fair value of BAM Surplus Notes			0.0	15.8
Unrealized loss on deconsolidation of BAM		(114.5)	0.0	(114.5)
Commission and fee revenues	82.3	46.8	193.7	108.4
Other revenues	73.5	20.1	155.6	57.3
Total revenues	864.2	839.1	2,131.2	1,881.8
Loss and LAE	258.5	296.9	686.8	666.6
Acquisition expenses	111.8	106.4	308.4	253.9
Cost of sales	55.3	7.6	105.2	22.2
Broker commission expenses	27.0	15.9	62.3	37.9
General and administrative expenses	176.8	121.9	410.3	369.6
Change in fair value of contingent consideration	36.1	34.2	74.2	47.5
Interest expense	19.9	16.7	56.3	46.7
Total expenses	685.4	599.6	1,703.5	1,444.4
Pre-tax income (loss)	178.8	239.5	427.7	437.4
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	2.9	11.2	19.1	19.8
Net investment income	6.2	9.6	24.5	27.9
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes				0.0
Unrealized loss on deconsolidation of BAM		0.0		0.0
Commission and fee revenues	4.3	4.1	12.4	11.1
Other revenues	69.3	14.8	139.7	43.7
Total revenues	105.3	157.6	246.8	322.6
Loss and LAE	0.6	4.2	18.8	8.1
Acquisition expenses	1.0	4.5	7.0	7.1
Cost of sales	55.3	7.6	105.2	22.2
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	72.4	32.9	161.7	126.4
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	1.0	0.3	2.3	1.6
Total expenses	130.3	49.5	295.0	165.4
Pre-tax income (loss)	(25.0)	108.1	(48.2)	157.2
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	76.2	135.6	249.6	230.7
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	14.9	29.7	49.5	60.4
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	7.7	88.2	1.6	159.7
Change in fair value of BAM Surplus Notes		15.8
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	7.7	88.2	1.6	159.7
Change in fair value of BAM Surplus Notes		0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and LAE	254.6	288.3	652.1	644.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	542.2	552.2	1,264.4	1,173.3
Net investment income	27.2	24.3	77.0	66.5
Net realized and unrealized investment gains (losses)	17.8	53.2	98.4	84.1
Other revenues	2.3	3.7	10.8	9.6
Loss and LAE	254.6	288.3	652.1	644.0
Acquisition expenses	106.8	96.3	287.5	230.8
General and administrative expenses	61.7	68.9	154.1	154.0
Change in fair value of contingent consideration	36.1	34.2	74.2	47.5
Interest expense	4.4	4.9	12.9	15.0
Total expenses	463.6	492.6	1,180.8	1,091.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	498.6	507.2	1,201.7	1,110.5
Net investment income	24.8	21.3	70.2	57.6
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes				0.0
Unrealized loss on deconsolidation of BAM		0.0		0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	2.3	3.7	10.8	9.6
Total revenues	543.5	585.4	1,381.2	1,261.8
Loss and LAE	252.4	275.1	628.0	630.4
Acquisition expenses	92.4	83.3	272.0	212.9
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	61.7	68.9	154.0	153.9
Change in fair value of contingent consideration	36.1	34.2	74.2	47.5
Interest expense	4.4	4.9	12.9	15.0
Total expenses	447.0	466.4	1,141.1	1,059.7
Pre-tax income (loss)	96.5	119.0	240.1	202.1
Other underwriting expenses	33.6	40.3	108.4	103.9
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)	17.8	53.2	98.5	84.1
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	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 | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	43.6	45.0	62.7	62.8
Net investment income	2.4	3.0	6.8	8.9
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes				0.0
Unrealized loss on deconsolidation of BAM		0.0		0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total revenues	46.0	48.0	69.4	71.7
Loss and LAE	2.2	13.2	24.1	13.6
Acquisition expenses	14.4	13.0	15.5	17.9
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	0.0	0.0	0.1	0.1
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	16.6	26.2	39.7	31.6
Pre-tax income (loss)	29.4	21.8	29.7	40.1
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	(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	0.0	0.0
Change in fair value of BAM Surplus Notes		0.0
Financial Guarantee (HG Global) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	7.5	22.8	24.3
Net investment income	7.0	6.0	19.8	26.1
Net realized and unrealized investment gains (losses)	7.2	22.5	20.3	8.1
Interest income from BAM Surplus Notes	7.5	7.9	22.5	7.9
Change in fair value of BAM Surplus Notes	0.0	15.8	0.0	15.8
Unrealized loss on deconsolidation of BAM	0.0	(114.5)	0.0	(114.5)
Other revenues	0.1	0.0	0.2	1.1
Acquisition expenses	2.0	1.9	5.9	6.3
Broker commission expenses				0.0
General and administrative expenses	1.0	0.3	2.6	34.8
Interest expense	4.3	5.8	13.4	13.4
Total expenses	7.3	8.0	21.9	54.5
Financial Guarantee (HG Global) | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	7.5	22.8	21.5
Net investment income	7.0	6.0	19.8	17.3
Interest income from BAM Surplus Notes	7.5	7.9	22.5	21.1
Change in fair value of BAM Surplus Notes				15.8
Unrealized loss on deconsolidation of BAM		(114.5)		(114.5)
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.1	0.0	0.2	0.0
Total revenues	29.3	(54.8)	85.6	(25.6)
Loss and LAE	0.0	0.0	0.0	0.0
Acquisition expenses	2.0	1.9	5.9	5.9
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	1.0	0.3	2.6	1.3
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	4.3	5.8	13.4	13.4
Total expenses	7.3	8.0	21.9	20.6
Pre-tax income (loss)	22.0	(62.8)	63.7	(46.2)
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	7.2	22.5	20.3	13.2
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	0.0	0.0
Change in fair value of BAM Surplus Notes		15.8
Financial Guarantee (HG Global) | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums				2.8
Net investment income				8.8
Interest income from BAM Surplus Notes				(13.2)
Change in fair value of BAM Surplus Notes				0.0
Unrealized loss on deconsolidation of BAM				0.0
Commission and fee revenues				0.0
Other revenues				1.1
Total revenues				(5.6)
Loss and LAE				0.0
Acquisition expenses				0.4
Cost of sales				0.0
General and administrative expenses				33.5
Change in fair value of contingent consideration				0.0
Interest expense				0.0
Total expenses				33.9
Pre-tax income (loss)				(39.5)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)				(5.1)
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	0.0	0.0
Net investment income	18.0	17.2	56.7	50.1
Net realized and unrealized investment gains (losses)	36.1	29.5	80.9	77.5
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes				0.0
Unrealized loss on deconsolidation of BAM		0.0		0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.3	0.5	1.0	0.5
Total revenues	54.4	47.2	138.6	128.1
Loss and LAE	0.0	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0	0.0
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	4.7	3.6	12.3	10.5
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	6.1	5.7	18.6	16.7
Total expenses	10.8	9.3	30.9	27.2
Pre-tax income (loss)	43.6	37.9	107.7	100.9
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	36.1	29.5	80.9	77.5
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes		0.0
Bamboo | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	5.2	10.6	21.7	27.0
Net investment income	0.6	0.6	2.0	1.5
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes				0.0
Unrealized loss on deconsolidation of BAM		0.0		0.0
Commission and fee revenues	63.9	42.7	167.2	97.3
Other revenues	1.5	1.1	3.9	2.4
Total revenues	71.4	55.7	195.3	128.8
Loss and LAE	3.3	4.4	15.9	14.5
Acquisition expenses	2.0	3.7	8.0	9.7
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	21.5	15.9	56.8	37.9
General and administrative expenses	26.8	16.2	69.4	43.9
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	2.8	0.0	7.8	0.0
Total expenses	56.4	40.2	157.9	106.0
Pre-tax income (loss)	15.0	15.5	37.4	22.8
Amortization of other intangibles	4.0	3.9	12.0	12.4
Bamboo | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.2	0.7	0.5	0.6
Bamboo | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes		0.0
Distinguished | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0		0.0
Net investment income	0.2		0.2
Interest income from BAM Surplus Notes	0.0		0.0
Commission and fee revenues	14.1	0.0	14.1	0.0
Other revenues	0.2	0.0	0.2	0.0
Total revenues	14.3		14.3
Loss and LAE	0.0		0.0
Acquisition expenses	0.0		0.0
Cost of sales	0.0		0.0
Broker commission expenses	5.5	0.0	5.5	0.0
General and administrative expenses	10.2	0.0	10.2	0.0
Change in fair value of contingent consideration	0.0		0.0
Interest expense	1.3	0.0	1.3	0.0
Total expenses	17.0	$ 0.0	17.0	$ 0.0
Pre-tax income (loss)	(2.7)		(2.7)
Distinguished | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0		0.0
Distinguished | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0		0.0
Distinguished, net | Operating Segments
Segment Reporting Information [Line Items]
Other revenues	$ 0.0		$ 0.0