CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Revenues:
Earned insurance premiums	$ 1,440.9	$ 1,076.7	$ 664.2
Net investment income	194.2	124.4	82.5
Net realized and unrealized investment gains (losses)	434.2	(191.5)	(246.1)
Commission revenues	13.2	11.5	9.6
Other revenues	84.2	136.8	104.2
Total revenues	2,166.7	1,157.9	614.4
Expenses:
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	290.1	250.6	186.3
Cost of sales	40.4	98.6	69.3
General and administrative expenses	432.6	364.1	291.6
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	62.7	40.3	20.5
Total expenses	1,601.3	1,307.3	888.0
Pre-tax income (loss) from continuing operations	565.4	(149.4)	(273.6)
Income tax (expense) benefit	15.5	(41.4)	(44.4)
Net income (loss) from continuing operations	580.9	(190.8)	(318.0)
Net income (loss) from discontinued operations, net of tax - NSM Group	0.0	16.4	(22.6)
Net income (loss)	580.9	712.4	(321.9)
Net (income) loss attributable to noncontrolling interests	(71.7)	80.4	46.5
Net income (loss) attributable to White Mountains’s common shareholders	509.2	792.8	(275.4)
NSM Group
Expenses:
Net gain (loss) from sale of discontinued operations, net of tax	0.0	886.8	0.0
Sirius Group
Expenses:
Net gain (loss) from sale of discontinued operations, net of tax	0.0	0.0	18.7
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	407.1	(98.5)	134.2
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
Other Operations
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	30.1	32.2	18.2
Commission revenues	13.2	11.5	9.6
Other revenues	80.5	127.2	90.7
Total revenues	339.4	76.3	(211.1)
Expenses:
Loss and loss adjustment expenses	0.0	0.0	0.0
Acquisition expenses	0.0	0.0	0.0
Cost of sales	40.4	98.6	69.3
General and administrative expenses	182.3	174.1	109.7
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	3.7	1.9	1.5
Total expenses	226.4	274.6	180.5
Pre-tax income (loss) from continuing operations	113.0	(198.3)	(391.6)
Other Operations | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	188.5	(1.6)	50.7
Other Operations | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	27.1	(93.0)	(380.3)
HG Global/BAM - Segment
Revenues:
Earned insurance premiums	31.2	33.3	26.9
HG Global/BAM - Segment | Operating Segments
Revenues:
Earned insurance premiums	31.2	33.3	26.9
Net investment income	31.7	21.5	17.5
Net realized and unrealized investment gains (losses)	26.6	(105.8)	(22.9)
Other revenues	2.9	4.6	1.5
Total revenues	92.4	(46.4)	23.0
Expenses:
Acquisition expenses	8.6	11.2	8.3
General and administrative expenses	68.9	69.1	57.1
Interest expense	16.5	8.3	0.0
Total expenses	94.0	88.6	65.4
Ark/WM Outrigger
Revenues:
Earned insurance premiums	1,409.7	1,043.4	637.3
Expenses:
Loss and loss adjustment expenses	726.8	536.4	314.8
Ark/WM Outrigger | Operating Segments
Revenues:
Earned insurance premiums	1,409.7	1,043.4	637.3
Net investment income	61.4	16.3	2.9
Net realized and unrealized investment gains (losses)	85.9	(55.2)	16.5
Other revenues	0.8	5.0	11.8
Total revenues	1,557.8	1,009.5	668.5
Expenses:
Loss and loss adjustment expenses	726.8	536.4	314.8
Acquisition expenses	281.5	239.4	178.0
General and administrative expenses	162.0	106.2	110.0
Change in fair value of contingent consideration	48.7	17.3	5.5
Interest expense	21.3	15.1	7.3
Total expenses	1,240.3	914.4	615.6
Kudu | Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	71.0	54.4	43.9
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Commission revenues	0.0	0.0	0.0
Other revenues	0.0	0.0	0.2
Total revenues	177.1	118.5	134.0
Expenses:
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	19.4	14.7	14.8
Change in fair value of contingent consideration	0.0	0.0	0.0
Interest expense	21.2	15.0	11.7
Total expenses	40.6	29.7	26.5
Pre-tax income (loss) from continuing operations	136.5	88.8	107.5
Kudu | Operating Segments | Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	106.1	64.1	89.9
Kudu | Operating Segments | MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0	$ 0.0	$ 0.0