Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 343.9	$ 301.0	$ 662.9	$ 563.8
Net investment income	57.4	43.0	114.4	82.0
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(60.2)	11.4	166.6	209.5
Commission and fee 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
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
Broker commission expenses	12.7		22.0
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
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	8.6	0.0	8.6	0.0
Net investment income	8.4	7.0	18.3	14.0
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Commission and fee 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
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
Broker commission expenses	0.0		0.0
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
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	79.0	88.7	95.1	201.6
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	8.5	76.0	30.7	117.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(139.2)	(77.3)	71.5	7.9
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(139.2)	(77.3)	71.5	7.9
Financial Guarantee (HG Global/BAM) | Operating Segments
Segment Reporting Information [Line Items]
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
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
Financial Guarantee (HG Global/BAM) | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.5	6.4	14.0	12.8
Net investment income	5.9	4.1	11.3	8.1
Net investment income (expense) - BAM Surplus Note interest	6.6	6.5	13.2	13.1
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total revenues	18.0	11.3	29.2	36.2
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	2.2	1.8	4.0	3.6
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	0.6	0.3	1.0	1.4
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	4.1	2.5	7.6	7.0
Total expenses	6.9	4.6	12.6	12.0
Pre-tax income (loss)	11.1	6.7	16.6	24.2
Financial Guarantee (HG Global/BAM) | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(2.0)	(5.7)	(9.3)	2.2
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	0.0	0.0
Financial Guarantee (HG Global/BAM) | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	1.5	1.3	2.8	2.6
Net investment income	4.5	3.5	8.8	6.7
Net investment income (expense) - BAM Surplus Note interest	(6.6)	(6.5)	(13.2)	(13.1)
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.6	0.5	1.1	1.3
Total revenues	(2.3)	(5.4)	(5.6)	2.4
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	0.0	(0.3)	0.4	0.6
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	16.6	14.6	33.5	30.8
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	16.6	14.3	33.9	31.4
Pre-tax income (loss)	(18.9)	(19.7)	(39.5)	(29.0)
Financial Guarantee (HG Global/BAM) | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(2.3)	(4.2)	(5.1)	4.9
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	0.0	0.0
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	175.7	167.5	355.7	315.3
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
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
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
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | UNITED KINGDOM
Segment Reporting Information [Line Items]
Earned insurance premiums	193.7	168.8	371.1	314.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | BERMUDA
Segment Reporting Information [Line Items]
Earned insurance premiums	117.1	114.9	232.2	218.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	310.8	283.7	603.3	533.6
Net investment income	19.3	11.2	36.3	19.6
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	2.4	(2.0)	5.9	(4.7)
Total revenues	352.8	310.9	676.4	591.0
Loss and loss adjustment expenses	176.0	167.1	355.3	314.7
Acquisition expenses	65.9	59.4	129.6	118.3
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	42.8	34.9	85.0	70.0
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	302.7	268.8	593.3	513.0
Pre-tax income (loss)	50.1	42.1	83.1	78.0
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)	20.3	18.0	30.9	42.5
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	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	7.5	9.6	17.8	14.8
Net investment income	3.0	2.5	5.9	4.7
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total revenues	10.5	12.1	23.7	19.5
Loss and loss adjustment expenses	(0.3)	0.4	0.4	0.6
Acquisition expenses	2.3	2.0	4.9	2.9
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0		0.0
General and administrative expenses	0.1	0.0	0.1	0.1
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	2.1	2.4	5.4	3.6
Pre-tax income (loss)	8.4	9.7	18.3	15.9
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	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	0.0	0.0
Asset Management (Kudu) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	15.7	14.7	32.9	28.9
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	54.5	4.6	48.0	34.2
Commission and fee 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
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
Broker commission expenses	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
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	54.5	4.6	48.0	34.2
Asset Management (Kudu) | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0	0.0	0.0
Bamboo | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	8.0	0.0	16.4	0.0
Net investment income	0.6		0.9
Net investment income (expense) - BAM Surplus Note interest	0.0		0.0
Commission and fee 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
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
Broker commission expenses	12.7	0.0	22.0	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
Bamboo | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0		(0.1)
Bamboo | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0		0.0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items | Operating Segments
Segment Reporting Information [Line Items]
Other revenues	$ 0.7		$ 1.3