CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Revenues:
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
Expenses:
Broker commission expenses	27.0	15.9	62.3	37.9
Loss and loss adjustment expenses	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
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
Income tax (expense) benefit	(17.1)	(11.6)	(39.6)	(28.9)
Net income (loss)	161.7	227.9	388.1	408.5
Net (income) loss attributable to noncontrolling interests	(47.9)	(48.9)	(117.5)	(47.7)
Net income (loss) attributable to White Mountains’s common shareholders	113.8	179.0	270.6	360.8
Other Operations
Revenues:
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
Expenses:
Broker commission expenses	0.0	0.0	0.0	0.0
Loss and loss adjustment expenses	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
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
Revenues:
Net realized and unrealized investment gains (losses)	76.2	135.6	249.6	230.7
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	14.9	29.7	49.5	60.4
MediaAlpha
Revenues:
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
Revenues:
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)
Expenses:
Loss and loss adjustment expenses	254.6	288.3	652.1	644.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
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
Total revenues	589.5	633.4	1,450.6	1,333.5
Expenses:
Loss and loss adjustment expenses	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
Financial Guarantee (HG Global) | Operating Segments
Revenues:
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
Total revenues	29.3	(54.8)	85.6	(31.2)
Expenses:
Broker commission expenses				0.0
Acquisition expenses	2.0	1.9	5.9	6.3
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
Asset Management (Kudu) | Operating Segments
Revenues:
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
Expenses:
Broker commission expenses	0.0	0.0	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	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
Revenues:
Net realized and unrealized investment gains (losses)	36.1	29.5	80.9	77.5
Asset Management (Kudu) | MediaAlpha | Operating Segments
Revenues:
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 Distribution (Bamboo)
Expenses:
Loss and loss adjustment expenses	3.3	4.4	15.9	14.5
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	5.2	10.6	21.7	27.0
Commission and fee revenues	63.9	42.7	167.2	97.3
Other revenues	2.3	2.4	6.4	4.5
Total revenues	71.4	55.7	195.3	128.8
Expenses:
Broker commission expenses	21.5	15.9	56.8	37.9
Loss and loss adjustment expenses	3.3	4.4	15.9	14.5
Acquisition expenses	2.0	3.7	8.0	9.7
General and administrative expenses	26.8	16.2	69.4	43.9
Interest expense	2.8	0.0	7.8	0.0
Total expenses	56.4	40.2	157.9	106.0
Specialty Insurance Distribution (Distinguished) | Operating Segments
Revenues:
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	0.0	14.3	0.0
Expenses:
Broker commission expenses	5.5	0.0	5.5	0.0
Loss and loss adjustment expenses	0.0		0.0
Acquisition expenses	0.0		0.0
Cost of sales	0.0		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)
Specialty Insurance Distribution (Distinguished) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0.0		0.0
Specialty Insurance Distribution (Distinguished) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0		$ 0.0