CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Revenues:
Earned insurance premiums	$ 1,691.6	$ 1,440.9	$ 1,076.7
Net investment income	227.4	194.2	124.4
Net realized and unrealized investment gains (losses)	184.9	434.2	(191.5)
Interest income from BAM Surplus Notes	15.8	0.0	0.0
Change in fair value of BAM Surplus Notes	0.5	0.0	0.0
Unrealized loss on deconsolidation of BAM	(114.5)	0.0	0.0
Commission and fees revenues	149.4	13.2	11.5
Other revenues	84.7	84.2	136.8
Total revenues	2,239.8	2,166.7	1,157.9
Expenses:
Broker commission expense	51.3
Loss and loss adjustment expenses	888.5	726.8	536.4
Acquisition expenses	341.5	290.1	250.6
Cost of sales	29.6	40.4	98.6
General and administrative expenses	490.1	432.6	364.1
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	60.8	62.7	40.3
Total expenses	1,923.1	1,601.3	1,307.3
Pre-tax income (loss) from continuing operations	316.7	565.4	(149.4)
Income tax (expense) benefit	(32.6)	15.5	(41.4)
Net income (loss) from continuing operations	284.1	580.9	(190.8)
Net income (loss) from discontinued operations, net of tax - NSM Group	0.0	0.0	16.4
Net income (loss)	284.1	580.9	712.4
Net (income) loss attributable to noncontrolling interests	(53.7)	(71.7)	80.4
Net income (loss) attributable to White Mountains’s common shareholders	230.4	509.2	792.8
NSM Group
Expenses:
Net income (loss) from discontinued operations, net of tax - NSM Group	0.0	0.0	16.4
Net gain (loss) from sale of discontinued operations, net of tax - NSM Group	0.0	0.0	886.8
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	146.9	407.1	(98.5)
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	38.0	27.1	(93.0)
Other Operations
Revenues:
Earned insurance premiums	32.7	0.0	0.0
Net investment income	35.6	30.1	32.2
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 fees revenues	14.8	13.2	11.5
Other revenues	56.8	80.5	127.2
Total revenues	234.9	339.4	76.3
Expenses:
Broker commission expense	0.0
Loss and loss adjustment expenses	12.1	0.0	0.0
Acquisition expenses	12.1	0.0	0.0
Cost of sales	29.6	40.4	98.6
General and administrative expenses	169.5	182.3	174.1
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	2.5	3.7	1.9
Total expenses	225.8	226.4	274.6
Pre-tax income (loss) from continuing operations	9.1	113.0	(198.3)
Other Operations | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	57.0	188.5	(1.6)
Other Operations | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	38.0	27.1	(93.0)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Revenues:
Earned insurance premiums	1,587.8	1,409.7	1,043.4
Expenses:
Loss and loss adjustment expenses	855.8	726.8	536.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	1,587.8	1,409.7	1,043.4
Net investment income	90.7	61.4	16.3
Net realized and unrealized investment gains (losses)	50.1	85.9	(55.2)
Other revenues	22.3	0.8	5.0
Total revenues	1,750.9	1,557.8	1,009.5
Expenses:
Loss and loss adjustment expenses	855.8	726.8	536.4
Acquisition expenses	307.1	281.5	239.4
General and administrative expenses	208.4	162.0	106.2
Change in fair value of contingent consideration	61.3	48.7	17.3
Interest expense	19.5	21.3	15.1
Total expenses	1,452.1	1,240.3	914.4
Financial Guarantee (HG Global)
Revenues:
Earned insurance premiums	31.7	31.2	33.3
Financial Guarantee (HG Global) | Operating Segments
Revenues:
Earned insurance premiums	31.7	31.2	33.3
Net investment income	32.2	31.7	21.5
Net realized and unrealized investment gains (losses)	(11.5)	26.6	(105.8)
Interest income from BAM Surplus Notes	15.8	0.0	0.0
Change in fair value of BAM Surplus Notes	0.5	0.0	0.0
Unrealized loss on deconsolidation of BAM	(114.5)	0.0	0.0
Other revenues	1.2	2.9	4.6
Total revenues	(44.6)	92.4	(46.4)
Expenses:
Broker commission expense	0.0
Acquisition expenses	8.2	8.6	11.2
General and administrative expenses	35.7	68.9	69.1
Interest expense	16.7	16.5	8.3
Total expenses	60.6	94.0	88.6
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	66.7	71.0	54.4
Net realized and unrealized investment gains (losses)	51.3	106.1	64.1
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 fees revenues	0.0	0.0	0.0
Other revenues	0.8	0.0	0.0
Total revenues	118.8	177.1	118.5
Expenses:
Broker commission expense	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	15.4	19.4	14.7
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	22.1	21.2	15.0
Total expenses	37.5	40.6	29.7
Pre-tax income (loss) from continuing operations	81.3	136.5	88.8
Asset Management (Kudu) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	51.3	106.1	64.1
Asset Management (Kudu) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0
P&C Insurance Distribution (Bamboo)
Expenses:
Loss and loss adjustment expenses	20.6
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	39.4	0.0	0.0
Net investment income	2.2
Interest income from BAM Surplus Notes	0.0
Change in fair value of BAM Surplus Notes	0.0
Unrealized loss on deconsolidation of BAM	0.0
Commission and fees revenues	134.6	0.0	0.0
Other revenues	5.8	0.0	0.0
Total revenues	179.8	0.0	0.0
Expenses:
Broker commission expense	51.3	0.0	0.0
Loss and loss adjustment expenses	20.6	0.0	0.0
Acquisition expenses	14.1	0.0	0.0
Cost of sales	0.0
General and administrative expenses	61.1	0.0	0.0
Change in fair value of contingent consideration	0.0
Interest expense	0.0
Total expenses	147.1	$ 0.0	$ 0.0
Pre-tax income (loss) from continuing operations	32.7
P&C Insurance Distribution (Bamboo) | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	0.0
P&C Insurance Distribution (Bamboo) | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0