CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Revenues:
Earned insurance premiums	$ 319.0	$ 262.8
Net investment income	57.0	39.0
Net realized and unrealized investment gains (losses)	226.8	198.1
Commission revenues	25.5	3.3
Other revenues	19.0	28.7
Total revenues	647.3	531.9
Expenses:
Loss and loss adjustment expenses	185.8	147.8
Acquisition expenses	71.6	62.5
Cost of sales	7.6	13.9
General and administrative expenses	125.2	96.0
Broker commission expenses	9.3
Change in fair value of contingent consideration	0.0	(2.4)
Interest expense	15.2	15.0
Total expenses	414.7	332.8
Pre-tax income (loss)	232.6	199.1
Income tax (expense) benefit	(10.8)	(11.9)
Net income (loss)	221.8	187.2
Net (income) loss attributable to noncontrolling interests	14.6	(7.7)
Net income (loss) attributable to White Mountains’s common shareholders	236.4	179.5
Other Operations
Revenues:
Earned insurance premiums	0.0	0.0
Net investment income	9.9	7.0
Commission revenues	3.6	3.3
Other revenues	14.4	30.6
Total revenues	260.8	167.9
Expenses:
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	0.0	0.0
Cost of sales	7.6	13.9
General and administrative expenses	50.3	39.7
Broker commission expenses	0.0
Change in fair value of contingent consideration	0.0	0.0
Interest expense	0.7	0.8
Total expenses	58.6	54.4
Pre-tax income (loss)	202.2	113.5
Investments, Excluding MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	16.1	112.9
Investments, Excluding MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	22.2	41.8
MediaAlpha
Revenues:
Net realized and unrealized investment gains (losses)	210.7	85.2
MediaAlpha | Other Operations
Revenues:
Net realized and unrealized investment gains (losses)	210.7	85.2
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Expenses:
Loss and loss adjustment expenses	180.0	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Revenues:
Earned insurance premiums	302.8	255.1
Net investment income	19.9	10.6
Net realized and unrealized investment gains (losses)	10.6	24.5
Other revenues	3.5	(2.7)
Total revenues	336.8	287.5
Expenses:
Loss and loss adjustment expenses	180.0	147.8
Acquisition expenses	66.3	59.8
General and administrative expenses	42.2	35.2
Change in fair value of contingent consideration	0.0	(2.4)
Interest expense	5.4	5.0
Total expenses	293.9	245.4
Financial Guarantee (HG Global/BAM) | Operating Segments
Revenues:
Earned insurance premiums	7.8	7.7
Net investment income	9.7	7.2
Net realized and unrealized investment gains (losses)	(10.1)	17.0
Other revenues	0.5	0.8
Total revenues	7.9	32.7
Expenses:
Acquisition expenses	2.2	2.7
General and administrative expenses	17.3	17.3
Interest expense	3.5	4.5
Total expenses	23.0	24.5
Asset Management (Kudu) | Operating Segments
Revenues:
Earned insurance premiums	0.0	0.0
Net investment income	17.2	14.2
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Commission revenues	0.0	0.0
Other revenues	0.0	0.0
Total revenues	10.7	43.8
Expenses:
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	3.4	3.8
Broker commission expenses	0.0
Change in fair value of contingent consideration	0.0	0.0
Interest expense	5.6	4.7
Total expenses	9.0	8.5
Pre-tax income (loss)	1.7	35.3
Asset Management (Kudu) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Asset Management (Kudu) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	0.0	0.0
P&C Insurance Distribution (Bamboo) | Operating Segments
Revenues:
Earned insurance premiums	8.4	0.0
Net investment income	0.3
Commission revenues	21.9	0.0
Other revenues	0.8	0.0
Total revenues	31.1	0.0
Expenses:
Loss and loss adjustment expenses	5.8	0.0
Acquisition expenses	3.1	0.0
Cost of sales	0.0
General and administrative expenses	12.0	0.0
Broker commission expenses	9.3	0.0
Change in fair value of contingent consideration	0.0
Interest expense	0.0
Total expenses	30.2	$ 0.0
Pre-tax income (loss)	0.9
P&C Insurance Distribution (Bamboo) | Investments, Excluding MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	(0.1)
P&C Insurance Distribution (Bamboo) | MediaAlpha | Operating Segments
Revenues:
Net realized and unrealized investment gains (losses)	$ 0.0