CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Revenues:
Earned insurance premiums	$ 581.5	$ 506.8	$ 1,244.4	$ 1,070.6
Net investment income	57.7	48.1	172.1	130.1
Net realized and unrealized investment gains (losses)	223.8	(58.3)	390.4	151.2
Interest Income, Operating	(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 fees 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
Expenses:
Broker commission expenses	15.9		37.9
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
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
Income tax (expense) benefit	(11.6)	(7.3)	(28.9)	(19.4)
Net income (loss)	227.9	13.8	408.5	218.2
Net (income) loss attributable to noncontrolling interests	(48.9)	9.8	(47.7)	4.5
Net income (loss) attributable to White Mountains’s common shareholders	179.0	23.6	360.8	222.7
Other Operations
Revenues:
Earned insurance premiums	11.2	0.0	19.8	0.0
Net investment income	9.6	8.0	27.9	22.0
Interest Income, Operating	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 fees 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
Expenses:
Broker commission expenses	0.0		0.0
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
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
Revenues:
Net realized and unrealized investment gains (losses)	135.6	(11.5)	230.7	190.1
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	29.7	8.0	60.4	125.8
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	88.2	(46.8)	159.7	(38.9)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Expenses:
Loss and loss adjustment expenses	288.3	265.8	644.0	581.1
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
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
Expenses:
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
Financial Guarantee (HG Global) | Operating Segments
Revenues:
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, Operating	(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
Expenses:
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
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	17.2	15.1	50.1	44.0
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Interest Income, Operating	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 fees 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
Expenses:
Broker commission expenses	0.0		0.0
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
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
Revenues:
Net realized and unrealized investment gains (losses)	29.5	11.2	77.5	45.4
Asset Management (Kudu) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0	0.0
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	10.6	0.0	27.0	0.0
Net investment income	0.6		1.5
Interest Income, Operating	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 fees 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
Expenses:
Broker commission expenses	15.9	0.0	37.9	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
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
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0.7		0.6
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0		$ 0.0