CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Revenues:
Earned insurance premiums	$ 1,777.1	$ 1,691.6	$ 1,440.9
Net investment income	248.0	227.4	194.2
Net realized and unrealized investment gains (losses)	361.5	184.9	434.2
Interest income from BAM Surplus Notes	29.8	15.8	0.0
Change in fair value of BAM Surplus Notes	(37.5)	0.5	0.0
Unrealized loss on deconsolidation of BAM	0.0	(114.5)	0.0
Commission and fee revenues	284.5	149.4	13.2
Net gain on sale of the Bamboo Group	849.3	0.0	0.0
Other revenues	222.3	84.7	84.2
Total revenues	3,735.0	2,239.8	2,166.7
Expenses:
Broker commission expense	94.1	51.3
Loss and loss adjustment expenses	882.2	888.5	726.8
Acquisition expenses	420.2	341.5	290.1
Cost of sales	151.8	29.6	40.4
General and administrative expenses	606.6	490.1	432.6
Change in fair value of contingent consideration	173.0	61.3	48.7
Interest expense	78.4	60.8	62.7
Total expenses	2,406.3	1,923.1	1,601.3
Pre-tax income (loss)	1,328.7	316.7	565.4
Income tax (expense) benefit	(126.9)	(32.6)	15.5
Net income (loss)	1,201.8	284.1	580.9
Net (income) loss attributable to noncontrolling interests	(95.4)	(53.7)	(71.7)
Net income (loss) attributable to White Mountains’s common shareholders	1,106.4	230.4	509.2
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	331.9	146.9	407.1
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	29.6	38.0	27.1
Change in fair value of BAM Surplus Notes		0.5
Other Operations
Revenues:
Earned insurance premiums	22.2	32.7	0.0
Net investment income	32.1	35.6	30.1
Interest income 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 fee revenues	16.4	14.8	13.2
Net gain on sale of the Bamboo Group	849.3	0.0	0.0
Other revenues	200.6	56.8	80.5
Total revenues	1,230.2	234.9	339.4
Expenses:
Broker commission expense	0.0	0.0
Loss and loss adjustment expenses	20.0	12.1	0.0
Acquisition expenses	8.3	12.1	0.0
Cost of sales	151.8	29.6	40.4
General and administrative expenses	236.7	169.5	182.3
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	3.2	2.5	3.7
Total expenses	420.0	225.8	226.4
Pre-tax income (loss)	810.2	9.1	113.0
Other Operations | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	80.0	57.0	188.5
Other Operations | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	29.6	38.0	27.1
Change in fair value of BAM Surplus Notes		0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Revenues:
Earned insurance premiums	1,697.4	1,587.8	1,409.7
Expenses:
Loss and loss adjustment expenses	844.2	855.8	726.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	1,697.4	1,587.8	1,409.7
Net investment income	106.5	90.7	61.4
Net realized and unrealized investment gains (losses)	124.6	50.1	85.9
Other revenues	15.3	22.3	0.8
Total revenues	1,943.8	1,750.9	1,557.8
Expenses:
Loss and loss adjustment expenses	844.2	855.8	726.8
Acquisition expenses	394.2	307.1	281.5
General and administrative expenses	205.1	208.4	162.0
Change in fair value of contingent consideration	173.0	61.3	48.7
Interest expense	17.1	19.5	21.3
Total expenses	1,633.6	1,452.1	1,240.3
Financial Guarantee (HG Global)
Revenues:
Earned insurance premiums	30.8	31.7	31.2
Financial Guarantee (HG Global) | Operating Segments
Revenues:
Earned insurance premiums	30.8	31.7	31.2
Net investment income	27.1	32.2	31.7
Net realized and unrealized investment gains (losses)	23.2	(11.5)	26.6
Interest income from BAM Surplus Notes	29.8	15.8	0.0
Change in fair value of BAM Surplus Notes	(37.5)	0.5	0.0
Unrealized loss on deconsolidation of BAM	0.0	(114.5)	0.0
Other revenues	0.2	1.2	2.9
Total revenues	73.6	(44.6)	92.4
Expenses:
Acquisition expenses	8.0	8.2	8.6
General and administrative expenses	3.1	35.7	68.9
Interest expense	17.4	16.7	16.5
Total expenses	28.5	60.6	94.0
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	78.7	66.7	71.0
Net realized and unrealized investment gains (losses)	103.5	51.3	106.1
Interest income 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 fee revenues	0.0	0.0	0.0
Net gain on sale of the Bamboo Group	0.0
Other revenues	1.2	0.8	0.0
Total revenues	183.4	118.8	177.1
Expenses:
Broker commission expense	0.0	0.0
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
General and administrative expenses	17.9	15.4	19.4
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	25.9	22.1	21.2
Total expenses	43.8	37.5	40.6
Pre-tax income (loss)	139.6	81.3	136.5
Asset Management (Kudu) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	103.5	51.3	106.1
Asset Management (Kudu) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
Change in fair value of BAM Surplus Notes		0.0
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	26.7	39.4	0.0
Net investment income	2.6	2.2
Interest income 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 fee revenues	211.4	134.6	0.0
Net gain on sale of the Bamboo Group	0.0
Other revenues	8.2	5.8	0.0
Total revenues	246.3	179.8	0.0
Expenses:
Broker commission expense	72.0	51.3	0.0
Loss and loss adjustment expenses	18.0	20.6	0.0
Acquisition expenses	9.7	14.1	0.0
Cost of sales	0.0	0.0
General and administrative expenses	96.8	61.1	0.0
Change in fair value of contingent consideration	0.0	0.0
Interest expense	9.7	0.0	0.0
Total expenses	206.2	147.1	0.0
Pre-tax income (loss)	40.1	32.7
P&C Insurance Distribution (Bamboo) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0.6	0.0
P&C Insurance Distribution (Bamboo) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0.0	0.0
Change in fair value of BAM Surplus Notes		0.0
Specialty Insurance Distribution (Distinguished) | Operating Segments
Revenues:
Earned insurance premiums	0.0
Net investment income	1.0
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0
Commission and fee revenues	56.7	0.0	0.0
Net gain on sale of the Bamboo Group	0.0
Other revenues	1.0	0.0	0.0
Total revenues	57.7	0.0	0.0
Expenses:
Broker commission expense	22.1	0.0	0.0
Loss and loss adjustment expenses	0.0
Acquisition expenses	0.0
Cost of sales	0.0
General and administrative expenses	47.0	0.0	0.0
Change in fair value of contingent consideration	0.0
Interest expense	5.1	0.0	0.0
Total expenses	74.2	$ 0.0	$ 0.0
Pre-tax income (loss)	(16.5)
Specialty Insurance Distribution (Distinguished) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0.0
Specialty Insurance Distribution (Distinguished) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0