Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 581.5	$ 506.8	$ 1,244.4	$ 1,070.6
Net investment income	57.7	48.1	172.1	130.1
Interest income (expense) from BAM Surplus Notes		0.0	7.9	0.0
Net realized and unrealized investment gains (losses)	223.8	(58.3)	390.4	151.2
Interest income from BAM Surplus Notes	7.9
Change in fair value of BAM Surplus Notes	15.8		15.8	0.0
Unrealized loss on deconsolidation of BAM	(114.5)		(114.5)	0.0
Commission and fee revenues	46.8	3.5	108.4	10.0
Other revenues	20.1	19.5	57.3	68.0
Total revenues	839.1	519.6	1,881.8	1,429.9
Loss and loss adjustment expenses	296.9	265.8	666.6	581.1
Acquisition expenses	106.4	92.4	253.9	217.8
Cost of sales	7.6	8.0	22.2	33.5
Broker commission expenses	15.9		37.9
General and administrative expenses	121.9	99.6	369.6	298.2
Change in fair value of contingent consideration	34.2	17.0	47.5	16.8
Interest expense	16.7	15.7	46.7	44.9
Total expenses	599.6	498.5	1,444.4	1,192.3
Pre-tax income (loss)	239.5	21.1	437.4	237.6
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	11.2	0.0	19.8	0.0
Net investment income	9.6	8.0	27.9	22.0
Interest income (expense) from BAM Surplus Notes		0.0	0.0	0.0
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0		0.0
Unrealized loss on deconsolidation of BAM	0.0		0.0
Commission and fee revenues	4.1	3.5	11.1	10.0
Other revenues	14.8	15.2	43.7	67.1
Total revenues	157.6	(12.1)	322.6	186.0
Loss and loss adjustment expenses	4.2	0.0	8.1	0.0
Acquisition expenses	4.5	0.0	7.1	0.0
Cost of sales	7.6	8.0	22.2	33.5
Broker commission expenses	0.0		0.0
General and administrative expenses	32.9	42.2	126.4	130.7
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.3	0.9	1.6	2.9
Total expenses	49.5	51.1	165.4	167.1
Pre-tax income (loss)	108.1	(63.2)	157.2	18.9
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	135.6	(11.5)	230.7	190.1
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	29.7	8.0	60.4	125.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
Financial Guarantee (HG Global) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	7.9	24.3	23.3
Net investment income	6.0	8.1	26.1	22.9
Net realized and unrealized investment gains (losses)	22.5	(24.1)	8.1	(17.0)
Interest income from BAM Surplus Notes	7.9	0.0	7.9	0.0
Change in fair value of BAM Surplus Notes	15.8	0.0	15.8	0.0
Unrealized loss on deconsolidation of BAM	(114.5)	0.0	(114.5)	0.0
Other revenues	0.0	0.7	1.1	2.0
Total revenues	(54.8)	(7.4)	(31.2)	31.2
Acquisition expenses	1.9	2.1	6.3	6.3
General and administrative expenses	0.3	17.5	34.8	49.7
Interest expense	5.8	3.8	13.4	10.8
Total expenses	8.0	23.4	54.5	66.8
Financial Guarantee (HG Global) | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	6.6	21.5	19.4
Net investment income	6.0	4.3	17.3	12.4
Interest income (expense) from BAM Surplus Notes		6.6	21.1	19.7
Interest income from BAM Surplus Notes	7.9
Change in fair value of BAM Surplus Notes	15.8		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	0.0	0.0	0.0	0.0
Total revenues	(54.8)	3.9	(25.6)	40.1
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	1.9	2.0	5.9	5.6
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	0.3	0.5	1.3	1.9
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	5.8	3.8	13.4	10.8
Total expenses	8.0	6.3	20.6	18.3
Pre-tax income (loss)	(62.8)	(2.4)	(46.2)	21.8
Financial Guarantee (HG Global) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	22.5	(13.6)	13.2	(11.4)
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
Financial Guarantee (HG Global) | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums		1.3	2.8	3.9
Net investment income		3.8	8.8	10.5
Interest income (expense) from BAM Surplus Notes		(6.6)	(13.2)	(19.7)
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
Other revenues		0.7	1.1	2.0
Total revenues		(11.3)	(5.6)	(8.9)
Loss and loss adjustment expenses		0.0	0.0	0.0
Acquisition expenses		0.1	0.4	0.7
Cost of sales		0.0	0.0	0.0
Broker commission expenses			0.0
General and administrative expenses		17.0	33.5	47.8
Change in fair value of contingent consideration		0.0	0.0	0.0
Interest expense		0.0	0.0	0.0
Total expenses		17.1	33.9	48.5
Pre-tax income (loss)		(28.4)	(39.5)	(57.4)
Financial Guarantee (HG Global) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)		(10.5)	(5.1)	(5.6)
Financial Guarantee (HG Global) | BAM | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)		0.0	0.0	0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	288.3	265.8	644.0	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	552.2	498.9	1,173.3	1,047.3
Net investment income	24.3	16.9	66.5	41.2
Net realized and unrealized investment gains (losses)	53.2	(6.6)	84.1	35.9
Other revenues	3.7	3.6	9.6	(1.1)
Total revenues	633.4	512.8	1,333.5	1,123.3
Loss and loss adjustment expenses	288.3	265.8	644.0	581.1
Acquisition expenses	96.3	90.3	230.8	211.5
General and administrative expenses	68.9	35.4	154.0	105.5
Change in fair value of contingent consideration	34.2	17.0	47.5	16.8
Interest expense	4.9	5.5	15.0	15.7
Total expenses	492.6	414.0	1,091.3	930.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | UNITED KINGDOM
Segment Reporting Information [Line Items]
Earned insurance premiums	326.4	273.6	697.5	588.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | BERMUDA
Segment Reporting Information [Line Items]
Earned insurance premiums	180.8	164.7	413.0	383.6
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	507.2	438.3	1,110.5	971.9
Net investment income	21.3	13.9	57.6	33.5
Interest income (expense) from BAM Surplus Notes		0.0	0.0	0.0
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0		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	3.7	3.6	9.6	(1.1)
Total revenues	585.4	449.2	1,261.8	1,040.2
Loss and loss adjustment expenses	275.1	258.5	630.4	573.2
Acquisition expenses	83.3	71.0	212.9	189.3
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	68.9	35.3	153.9	105.3
Change in fair value of contingent consideration	34.2	17.0	47.5	16.8
Interest expense	4.9	5.5	15.0	15.7
Total expenses	466.4	387.3	1,059.7	900.3
Pre-tax income (loss)	119.0	61.9	202.1	139.9
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)	53.2	(6.6)	84.1	35.9
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
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	45.0	60.6	62.8	75.4
Net investment income	3.0	3.0	8.9	7.7
Interest income (expense) from BAM Surplus Notes		0.0	0.0	0.0
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0		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	48.0	63.6	71.7	83.1
Loss and loss adjustment expenses	13.2	7.3	13.6	7.9
Acquisition expenses	13.0	19.3	17.9	22.2
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	0.0	0.1	0.1	0.2
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	26.2	26.7	31.6	30.3
Pre-tax income (loss)	21.8	36.9	40.1	52.8
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	0.0	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	0.0	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	17.2	15.1	50.1	44.0
Interest income (expense) from BAM Surplus Notes		0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0		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	0.5	0.0	0.5	0.0
Total revenues	47.2	26.3	128.1	89.4
Loss and loss adjustment expenses	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
General and administrative expenses	3.6	4.5	10.5	12.3
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	5.7	5.5	16.7	15.5
Total expenses	9.3	10.0	27.2	27.8
Pre-tax income (loss)	37.9	16.3	100.9	61.6
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
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
Bamboo | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	10.6	0.0	27.0	0.0
Net investment income	0.6		1.5
Interest income (expense) from BAM Surplus Notes			0.0
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0		0.0
Unrealized loss on deconsolidation of BAM	0.0		0.0
Commission and fee revenues	42.7	0.0	97.3	0.0
Other revenues	2.4	0.0	4.5	0.0
Total revenues	55.7	0.0	128.8	0.0
Loss and loss adjustment expenses	4.4	0.0	14.5	0.0
Acquisition expenses	3.7	0.0	9.7	0.0
Cost of sales	0.0		0.0
Broker commission expenses	15.9	0.0	37.9	0.0
General and administrative expenses	16.2	0.0	43.9	0.0
Change in fair value of contingent consideration	0.0		0.0
Interest expense	0.0		0.0
Total expenses	40.2	$ 0.0	106.0	$ 0.0
Pre-tax income (loss)	15.5		22.8
Bamboo | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.7		0.6
Bamboo | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0		0.0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments
Segment Reporting Information [Line Items]
Other revenues	$ 1.1		$ 2.4