CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Revenues:
Earned insurance premiums	$ 343.9	$ 301.0	$ 662.9	$ 563.8
Net investment income	57.4	43.0	114.4	82.0
Net realized and unrealized investment gains (losses)	(60.2)	11.4	166.6	209.5
Commission and fees revenues	36.1	3.2	61.6	6.5
Other revenues	18.2	19.8	37.2	48.5
Total revenues	395.4	378.4	1,042.7	910.3
Expenses:
Broker commission expenses	12.7		22.0
Loss and loss adjustment expenses	183.9	167.5	369.7	315.3
Acquisition expenses	75.9	62.9	147.5	125.4
Cost of sales	7.0	11.6	14.6	25.5
General and administrative expenses	122.5	102.6	247.7	198.6
Change in fair value of contingent consideration	13.3	2.2	13.3	(0.2)
Interest expense	14.8	14.2	30.0	29.2
Total expenses	430.1	361.0	844.8	693.8
Pre-tax income (loss)	(34.7)	17.4	197.9	216.5
Income tax (expense) benefit	(6.5)	(0.2)	(17.3)	(12.1)
Net income (loss)	(41.2)	17.2	180.6	204.4
Net (income) loss attributable to noncontrolling interests	(13.4)	2.4	1.2	(5.3)
Net income (loss) attributable to White Mountains’s common shareholders	(54.6)	19.6	181.8	199.1
Other Operations
Revenues:
Earned insurance premiums	8.6	0.0	8.6	0.0
Net investment income	8.4	7.0	18.3	14.0
Commission and fees revenues	3.4	3.2	7.0	6.5
Other revenues	14.5	21.3	28.9	51.9
Total revenues	(95.8)	30.2	165.0	198.1
Expenses:
Broker commission expenses	0.0		0.0
Loss and loss adjustment expenses	3.9	0.0	3.9	0.0
Acquisition expenses	2.6	0.0	2.6	0.0
Cost of sales	7.0	11.6	14.6	25.5
General and administrative expenses	43.2	48.8	93.5	88.5
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.6	1.2	1.3	2.0
Total expenses	57.3	61.6	115.9	116.0
Pre-tax income (loss)	(153.1)	(31.4)	49.1	82.1
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	79.0	88.7	95.1	201.6
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	8.5	76.0	30.7	117.8
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	(139.2)	(77.3)	71.5	7.9
MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	(139.2)	(77.3)	71.5	7.9
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Expenses:
Loss and loss adjustment expenses	175.7	167.5	355.7	315.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	318.3	293.3	621.1	548.4
Net investment income	22.3	13.7	42.2	24.3
Net realized and unrealized investment gains (losses)	20.3	18.0	30.9	42.5
Other revenues	2.4	(2.0)	5.9	(4.7)
Total revenues	363.3	323.0	700.1	610.5
Expenses:
Loss and loss adjustment expenses	175.7	167.5	355.7	315.3
Acquisition expenses	68.2	61.4	134.5	121.2
General and administrative expenses	42.9	34.9	85.1	70.1
Change in fair value of contingent consideration	13.3	2.2	13.3	(0.2)
Interest expense	4.7	5.2	10.1	10.2
Total expenses	304.8	271.2	598.7	516.6
Financial Guarantee (HG Global/BAM) | Operating Segments
Revenues:
Earned insurance premiums	9.0	7.7	16.8	15.4
Net investment income	10.4	7.6	20.1	14.8
Net realized and unrealized investment gains (losses)	(4.3)	(9.9)	(14.4)	7.1
Other revenues	0.6	0.5	1.1	1.3
Total revenues	15.7	5.9	23.6	38.6
Expenses:
Acquisition expenses	2.2	1.5	4.4	4.2
General and administrative expenses	17.2	14.9	34.5	32.2
Interest expense	4.1	2.5	7.6	7.0
Total expenses	23.5	18.9	46.5	43.4
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	15.7	14.7	32.9	28.9
Net realized and unrealized investment gains (losses)	54.5	4.6	48.0	34.2
Commission and fees revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total revenues	70.2	19.3	80.9	63.1
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	3.5	4.0	6.9	7.8
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	5.4	5.3	11.0	10.0
Total expenses	8.9	9.3	17.9	17.8
Pre-tax income (loss)	61.3	10.0	63.0	45.3
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	54.5	4.6	48.0	34.2
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	8.0	0.0	16.4	0.0
Net investment income	0.6		0.9
Commission and fees revenues	32.7	0.0	54.6	0.0
Other revenues	1.3	0.0	2.1	0.0
Total revenues	42.0	0.0	73.1	0.0
Expenses:
Broker commission expenses	12.7	0.0	22.0	0.0
Loss and loss adjustment expenses	4.3	0.0	10.1	0.0
Acquisition expenses	2.9	0.0	6.0	0.0
Cost of sales	0.0		0.0
General and administrative expenses	15.7	0.0	27.7	0.0
Change in fair value of contingent consideration	0.0		0.0
Interest expense	0.0		0.0
Total expenses	35.6	$ 0.0	65.8	$ 0.0
Pre-tax income (loss)	6.4		7.3
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0.0		(0.1)
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0		$ 0.0