Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 375.2	$ 343.9	$ 770.2	$ 662.9
Net investment income	61.4	57.4	121.0	114.4
Net realized and unrealized investment gains (losses)	117.3	(60.2)	167.3	166.6
Interest income from BAM Surplus Notes	7.5	0.0	15.0	0.0
Commission and fee revenues	63.3	36.1	111.4	61.6
Other revenues	64.5	18.2	82.1	37.2
Total revenues	689.2	395.4	1,267.0	1,042.7
Loss and loss adjustment expenses	166.5	183.9	428.3	369.7
Acquisition expenses	99.5	75.9	196.6	147.5
Cost of sales	42.4	7.0	49.9	14.6
Broker commission expenses	19.8	12.7	35.3	22.0
General and administrative expenses	137.5	122.5	233.5	247.7
Change in fair value of contingent consideration	28.4	13.3	38.1	13.3
Interest expense	18.6	14.8	36.4	30.0
Total expenses	512.7	430.1	1,018.1	844.8
Pre-tax income (loss)	176.5	(34.7)	248.9	197.9
UNITED KINGDOM
Segment Reporting Information [Line Items]
Earned insurance premiums	229.0	193.7	444.5	371.1
Commission and fee revenues	0.0	0.0	0.0	0.0
BERMUDA
Segment Reporting Information [Line Items]
Earned insurance premiums	144.6	148.7	309.2	289.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	2.3	8.6	16.2	8.6
Net investment income	8.6	8.4	18.3	18.3
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Commission and fee revenues	4.2	3.4	8.1	7.0
Other revenues	56.8	14.5	70.4	28.9
Total revenues	134.2	(95.8)	141.5	165.0
Loss and loss adjustment expenses	0.8	3.9	18.2	3.9
Acquisition expenses	0.9	2.6	6.0	2.6
Cost of sales	42.4	7.0	49.9	14.6
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	53.8	43.2	89.3	93.5
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.8	0.6	1.3	1.3
Total expenses	98.7	57.3	164.7	115.9
Pre-tax income (loss)	35.5	(153.1)	(23.2)	49.1
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	86.8	79.0	173.4	95.1
Investments, Excluding MediaAlpha \| Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	31.8	8.5	34.6	30.7
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	30.5	(139.2)	(6.1)	71.5
MediaAlpha \| Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	30.5	(139.2)	(6.1)	71.5
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Loss and loss adjustment expenses	164.0	175.7	397.5	355.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	364.2	318.3	722.2	621.1
Net investment income	26.3	22.3	49.8	42.2
Net realized and unrealized investment gains (losses)	51.1	20.3	80.6	30.9
Other revenues	6.3	2.4	8.5	5.9
Loss and loss adjustment expenses	164.0	175.7	397.5	355.7
Acquisition expenses	97.2	68.2	180.7	134.5
General and administrative expenses	56.5	42.9	92.4	85.1
Change in fair value of contingent consideration	28.4	13.3	38.1	13.3
Interest expense	4.3	4.7	8.5	10.1
Total expenses	350.4	304.8	717.2	598.7
P&C Insurance and Reinsurance (Ark/WM Outrigger) \| Ark \| Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	357.1	310.8	703.1	603.3
Net investment income	24.1	19.3	45.4	36.3
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	6.3	2.4	8.5	5.9
Total revenues	438.6	352.8	837.7	676.4
Loss and loss adjustment expenses	162.3	176.0	375.6	355.3
Acquisition expenses	95.8	65.9	179.6	129.6
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	56.5	42.8	92.3	85.0
Change in fair value of contingent consideration	28.4	13.3	38.1	13.3
Interest expense	4.3	4.7	8.5	10.1
Total expenses	347.3	302.7	694.1	593.3
Pre-tax income (loss)	91.3	50.1	143.6	83.1
Other underwriting expenses	46.3	33.1	74.8	63.6
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)	51.1	20.3	80.7	30.9
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	7.1	7.5	19.1	17.8
Net investment income	2.2	3.0	4.4	5.9
Interest income from BAM Surplus Notes	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	9.3	10.5	23.4	23.7
Loss and loss adjustment expenses	1.7	(0.3)	21.9	0.4
Acquisition expenses	1.4	2.3	1.1	4.9
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	0.0	0.1	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	3.1	2.1	23.1	5.4
Pre-tax income (loss)	6.2	8.4	0.3	18.3
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	(0.1)	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
Financial Guarantee (HG Global) \| Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.1	9.0	15.3	16.8
Net investment income	6.5	10.4	12.8	20.1
Net realized and unrealized investment gains (losses)	3.1	(4.3)	13.1	(14.4)
Interest income from BAM Surplus Notes	7.5	0.0	15.0	0.0
Other revenues	0.0	0.6	0.1	1.1
Acquisition expenses	2.0	2.2	3.9	4.4
Broker commission expenses				0.0
General and administrative expenses	1.0	17.2	1.6	34.5
Interest expense	4.5	4.1	9.1	7.6
Total expenses	7.5	23.5	14.6	46.5
Financial Guarantee (HG Global) \| HG Global \| Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.1	7.5	15.3	14.0
Net investment income	6.5	5.9	12.8	11.3
Interest income from BAM Surplus Notes	7.5	6.6	15.0	13.2
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.1	0.0
Total revenues	24.2	18.0	56.3	29.2
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	2.0	2.2	3.9	4.0
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	0.0	0.0	0.0	0.0
General and administrative expenses	1.0	0.6	1.6	1.0
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	4.5	4.1	9.1	7.6
Total expenses	7.5	6.9	14.6	12.6
Pre-tax income (loss)	16.7	11.1	41.7	16.6
Financial Guarantee (HG Global) \| HG Global \| Investments, Excluding MediaAlpha \| Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	3.1	(2.0)	13.1	(9.3)
Financial Guarantee (HG Global) \| 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 \| Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums		1.5		2.8
Net investment income		4.5		8.8
Interest income from BAM Surplus Notes		(6.6)		(13.2)
Commission and fee revenues		0.0		0.0
Other revenues		0.6		1.1
Total revenues		(2.3)		(5.6)
Loss and loss adjustment expenses		0.0		0.0
Acquisition expenses		0.0		0.4
Cost of sales		0.0		0.0
Broker commission expenses		0.0
General and administrative expenses		16.6		33.5
Change in fair value of contingent consideration		0.0		0.0
Interest expense		0.0		0.0
Total expenses		16.6		33.9
Pre-tax income (loss)		(18.9)		(39.5)
Financial Guarantee (HG Global) \| BAM \| Investments, Excluding MediaAlpha \| Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)		(2.3)		(5.1)
Financial Guarantee (HG Global) \| 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	0.0	0.0
Net investment income	19.3	15.7	38.7	32.9
Net realized and unrealized investment gains (losses)	0.8	54.5	44.8	48.0
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Commission and fee revenues	0.0	0.0	0.0	0.0
Other revenues	0.3	0.0	0.7	0.0
Total revenues	20.4	70.2	84.2	80.9
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	0.0	0.0
General and administrative expenses	3.6	3.5	7.6	6.9
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	6.1	5.4	12.5	11.0
Total expenses	9.7	8.9	20.1	17.9
Pre-tax income (loss)	10.7	61.3	64.1	63.0
Asset Management (Kudu) \| Investments, Excluding MediaAlpha \| Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.8	54.5	44.8	48.0
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	1.6	8.0	16.5	16.4
Net investment income	0.7	0.6	1.4	0.9
Interest income from BAM Surplus Notes	0.0	0.0	0.0	0.0
Commission and fee revenues	59.1	32.7	103.3	54.6
Other revenues	1.1
Total revenues	62.5	42.0	123.9	73.1
Loss and loss adjustment expenses	1.7	4.3	12.6	10.1
Acquisition expenses	(0.6)	2.9	6.0	6.0
Cost of sales	0.0	0.0	0.0	0.0
Broker commission expenses	19.8	12.7	35.3	22.0
General and administrative expenses	22.6	15.7	42.6	27.7
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	2.9	0.0	5.0	0.0
Total expenses	46.4	35.6	101.5	65.8
Pre-tax income (loss)	16.1	6.4	22.4	7.3
Amortization of other intangibles	4.0	4.3	8.0	8.5
Bamboo \| Investments, Excluding MediaAlpha \| Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	0.0	0.0	0.3	(0.1)
Bamboo \| MediaAlpha \| Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	$ 0.0	0.0	0.0	0.0
Bamboo Ide8 Services, LLC - Segment, Net Of Reconciling Items \| Operating Segments
Segment Reporting Information [Line Items]
Other revenues		$ 0.7	$ 2.4	$ 1.3

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