Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023	Sep. 30, 2022
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 506.8	$ 353.2	$ 1,070.6	$ 783.8
Net investment income	48.1	33.9	130.1	79.6
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	(58.3)	(48.0)	151.2	(255.2)
Commission revenues	3.5	3.2	10.0	8.7
Other revenues	19.5	40.9	68.0	103.4
Total revenues	519.6	383.2	1,429.9	720.3
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
Acquisition expenses	92.4	76.5	217.8	184.4
Cost of sales	8.0	25.0	33.5	68.8
General and administrative expenses	99.6	85.8	298.2	256.9
Change in fair value of contingent consideration	17.0	2.7	16.8	4.9
Interest expense	15.7	10.5	44.9	27.5
Total expenses	498.5	414.2	1,192.3	998.7
Pre-tax income (loss) from continuing operations	21.1	(31.0)	237.6	(278.4)
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	8.0	8.5	22.0	13.6
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Commission revenues	3.5	3.2	10.0	8.7
Other revenues	15.2	33.0	67.1	89.6
Total revenues	(12.1)	8.8	186.0	1.4
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	8.0	25.0	33.5	68.8
General and administrative expenses	42.2	41.3	130.7	122.1
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	0.9	0.6	2.9	1.2
Total expenses	51.1	66.9	167.1	192.1
Pre-tax income (loss) from continuing operations	(63.2)	(58.1)	18.9	(190.7)
Investments, Excluding MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(11.5)	(29.4)	190.1	(141.9)
Investments, Excluding MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	8.0	(17.3)	125.8	2.8
MediaAlpha
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
MediaAlpha | Other Operations
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(46.8)	(18.6)	(38.9)	(113.3)
HG Global/BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	7.9	7.1	23.3	26.0
Net investment income	8.1	5.7	22.9	15.1
Net realized and unrealized investment gains (losses)	(24.1)	(38.8)	(17.0)	(114.0)
Other revenues	0.7	1.3	2.0	3.7
Total revenues	(7.4)	(24.7)	31.2	(69.2)
Acquisition expenses	2.1	1.7	6.3	9.5
General and administrative expenses	17.5	15.8	49.7	49.5
Interest expense	3.8	2.0	10.8	5.4
Total expenses	23.4	19.5	66.8	64.4
HG Global/BAM | HG Global | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	6.6	5.9	19.4	21.5
Net investment income	4.3	2.8	12.4	7.1
Net investment income (expense) - BAM Surplus Note interest	6.6	2.9	19.7	8.8
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.1	0.0	0.3
Total revenues	3.9	(7.9)	40.1	(20.1)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	2.0	1.6	5.6	7.6
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	0.5	0.6	1.9	2.1
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	3.8	2.0	10.8	5.4
Total expenses	6.3	4.2	18.3	15.1
Pre-tax income (loss) from continuing operations	(2.4)	(12.1)	21.8	(35.2)
HG Global/BAM | HG Global | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(13.6)	(19.6)	(11.4)	(57.8)
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
HG Global/BAM | BAM | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	1.3	1.2	3.9	4.5
Net investment income	3.8	2.9	10.5	8.0
Net investment income (expense) - BAM Surplus Note interest	(6.6)	(2.9)	(19.7)	(8.8)
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	0.7	1.2	2.0	3.4
Total revenues	(11.3)	(16.8)	(8.9)	(49.1)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	0.1	0.1	0.7	1.9
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	17.0	15.2	47.8	47.4
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	17.1	15.3	48.5	49.3
Pre-tax income (loss) from continuing operations	(28.4)	(32.1)	(57.4)	(98.4)
HG Global/BAM | BAM | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	(10.5)	(19.2)	(5.6)	(56.2)
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	265.8	213.7	581.1	456.2
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	498.9	346.1	1,047.3	757.8
Net investment income	16.9	4.9	41.2	9.7
Net realized and unrealized investment gains (losses)	(6.6)	(14.4)	35.9	(76.5)
Other revenues	3.6	6.6	(1.1)	10.1
Total revenues	512.8	343.2	1,123.3	701.1
Loss and loss adjustment expenses	265.8	213.7	581.1	456.2
Acquisition expenses	90.3	74.8	211.5	174.9
General and administrative expenses	35.4	24.2	105.5	74.9
Change in fair value of contingent consideration	17.0	2.7	16.8	4.9
Interest expense	5.5	3.7	15.7	10.6
Total expenses	414.0	319.1	930.6	721.5
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	438.3	346.1	971.9	757.8
Net investment income	13.9	4.9	33.5	9.7
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	3.6	6.6	(1.1)	10.1
Total revenues	449.2	343.2	1,040.2	701.1
Loss and loss adjustment expenses	258.5	213.7	573.2	456.2
Acquisition expenses	71.0	74.8	189.3	174.9
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	35.3	24.2	105.3	74.9
Change in fair value of contingent consideration	17.0	2.7	16.8	4.9
Interest expense	5.5	3.7	15.7	10.6
Total expenses	387.3	319.1	900.3	721.5
Pre-tax income (loss) from continuing operations	61.9	24.1	139.9	(20.4)
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)	(6.6)	(14.4)	35.9	(76.5)
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	60.6		75.4
Net investment income	3.0		7.7
Net investment income (expense) - BAM Surplus Note interest	0.0		0.0
Commission revenues	0.0		0.0
Other revenues	0.0		0.0
Total revenues	63.6		83.1
Loss and loss adjustment expenses	7.3		7.9
Acquisition expenses	19.3		22.2
Cost of sales	0.0		0.0
General and administrative expenses	0.1		0.2
Change in fair value of contingent consideration	0.0		0.0
Interest expense	0.0		0.0
Total expenses	26.7		30.3
Pre-tax income (loss) from continuing operations	36.9		52.8
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
Kudu | Operating Segments
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	15.1	14.8	44.0	41.2
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	11.2	41.1	45.4	45.8
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.0	0.0	0.0
Total revenues	26.3	55.9	89.4	87.0
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	4.5	4.5	12.3	10.4
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	5.5	4.2	15.5	10.3
Total expenses	10.0	8.7	27.8	20.7
Pre-tax income (loss) from continuing operations	16.3	47.2	61.6	66.3
Kudu | Investments, Excluding MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	11.2	41.1	45.4	45.8
Kudu | MediaAlpha | Operating Segments
Segment Reporting Information [Line Items]
Net realized and unrealized investment gains (losses)	$ 0.0	$ 0.0	$ 0.0	$ 0.0