Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 319.0	$ 262.8
Net investment income	57.0	39.0
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	226.8	198.1
Commission and fee revenues	25.5	3.3
Other revenues	19.0	28.7
Total revenues	647.3	531.9
Loss and loss adjustment expenses	185.8	147.8
Acquisition expenses	71.6	62.5
Cost of sales	7.6	13.9
Broker commission expenses	9.3
General and administrative expenses	125.2	96.0
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
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	9.9	7.0
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Commission and fee revenues	3.6	3.3
Other revenues	14.4	30.6
Total revenues	260.8	167.9
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	0.0	0.0
Cost of sales	7.6	13.9
Broker commission expenses	0.0
General and administrative expenses	50.3	39.7
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
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	16.1	112.9
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	22.2	41.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	210.7	85.2
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	210.7	85.2
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	180.0	147.8
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
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
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
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | UNITED KINGDOM
Segment Reporting Information [Line Items]
Earned insurance premiums	177.4	146.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | BERMUDA
Segment Reporting Information [Line Items]
Earned insurance premiums	115.1	104.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	292.5	249.9
Net investment income	17.0	8.4
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Commission and fee revenues	0.0	0.0
Other revenues	3.5	(2.7)
Total revenues	323.6	280.1
Loss and loss adjustment expenses	179.3	147.6
Acquisition expenses	63.7	58.9
Cost of sales	0.0	0.0
Broker commission expenses	0.0
General and administrative expenses	42.2	35.1
Change in fair value of contingent consideration	0.0	(2.4)
Interest expense	5.4	5.0
Total expenses	290.6	244.2
Pre-tax income (loss)	33.0	35.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	10.6	24.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	10.3	5.2
Net investment income	2.9	2.2
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Commission and fee revenues	0.0	0.0
Other revenues	0.0	0.0
Total revenues	13.2	7.4
Loss and loss adjustment expenses	0.7	0.2
Acquisition expenses	2.6	0.9
Cost of sales	0.0	0.0
Broker commission expenses	0.0
General and administrative expenses	0.0	0.1
Change in fair value of contingent consideration	0.0	0.0
Interest expense	0.0	0.0
Total expenses	3.3	1.2
Pre-tax income (loss)	9.9	6.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger) | WM Outrigger Re | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0
Financial Guarantee (HG Global/BAM) | Operating Segments
Segment Reporting Information [Line Items]
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
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
Financial Guarantee (HG Global/BAM) | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	6.5	6.4
Net investment income	5.4	4.0
Net investment income (expense) - BAM Surplus Note interest	6.6	6.6
Commission and fee revenues	0.0	0.0
Other revenues	0.0	0.0
Total revenues	11.2	24.9
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	1.8	1.8
Cost of sales	0.0	0.0
Broker commission expenses	0.0
General and administrative expenses	0.4	1.1
Change in fair value of contingent consideration	0.0	0.0
Interest expense	3.5	4.5
Total expenses	5.7	7.4
Pre-tax income (loss)	5.5	17.5
Financial Guarantee (HG Global/BAM) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(7.3)	7.9
Financial Guarantee (HG Global/BAM) | HG Global | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0
Financial Guarantee (HG Global/BAM) | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	1.3	1.3
Net investment income	4.3	3.2
Net investment income (expense) - BAM Surplus Note interest	(6.6)	(6.6)
Commission and fee revenues	0.0	0.0
Other revenues	0.5	0.8
Total revenues	(3.3)	7.8
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	0.4	0.9
Cost of sales	0.0	0.0
Broker commission expenses	0.0
General and administrative expenses	16.9	16.2
Change in fair value of contingent consideration	0.0	0.0
Interest expense	0.0	0.0
Total expenses	17.3	17.1
Pre-tax income (loss)	(20.6)	(9.3)
Financial Guarantee (HG Global/BAM) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(2.8)	9.1
Financial Guarantee (HG Global/BAM) | BAM | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0
Asset Management (Kudu) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	17.2	14.2
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Commission and fee revenues	0.0	0.0
Other revenues	0.0	0.0
Total revenues	10.7	43.8
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	0.0	0.0
Cost of sales	0.0	0.0
Broker commission expenses	0.0
General and administrative expenses	3.4	3.8
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
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(6.5)	29.6
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0
Bamboo | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	8.4	0.0
Net investment income	0.3
Net investment income (expense) - BAM Surplus Note interest	0.0
Commission and fee revenues	21.9	0.0
Other revenues	0.8	0.0
Total revenues	31.1	0.0
Loss and loss adjustment expenses	5.8	0.0
Acquisition expenses	3.1	0.0
Cost of sales	0.0
Broker commission expenses	9.3	0.0
General and administrative expenses	12.0	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
Bamboo | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(0.1)
Bamboo | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments
Segment Reporting Information [Line Items]
Other revenues	$ 0.6