Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 262.8	$ 202.8
Net investment income	39.0	20.6
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	112.9	(8.4)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	85.2	18.8
Commission revenues	3.3	2.9
Other revenues	28.7	23.7
Total revenues	531.9	260.4
Loss and loss adjustment expenses	147.8	122.0
Acquisition expenses	62.5	52.9
Cost of sales	13.9	21.4
General and administrative expenses	96.0	69.9
Change in fair value of contingent consideration	(2.4)	2.1
Interest expense	15.0	6.9
Total expenses	332.8	275.2
Pre-tax income (loss) from continuing operations	199.1	(14.8)
Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Earned insurance premiums	7.7	8.4
Net investment income	7.2	4.6
Net realized and unrealized investment gains (losses)	17.0	(45.1)
Other revenues	0.8	0.8
Total revenues	32.7	(31.3)
Acquisition expenses	2.7	3.0
General and administrative expenses	17.3	16.3
Interest expense	4.5	0.0
Total expenses	24.5	19.3
Financial Guarantee (HG Global/BAM) | HG Global
Segment Reporting Information [Line Items]
Earned insurance premiums	6.4	6.9
Net investment income	4.0	2.1
Net investment income (expense) - BAM Surplus Note interest	6.6	2.9
Net realized and unrealized investment gains (losses)	7.9	(23.5)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
Commission revenues	0.0	0.0
Other revenues	0.0	0.1
Total revenues	24.9	(11.5)
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	1.8	2.6
Cost of sales	0.0	0.0
General and administrative expenses	1.1	0.7
Change in fair value of contingent consideration	0.0	0.0
Interest expense	4.5	0.0
Total expenses	7.4	3.3
Pre-tax income (loss) from continuing operations	17.5	(14.8)
Financial Guarantee (HG Global/BAM) | BAM
Segment Reporting Information [Line Items]
Earned insurance premiums	1.3	1.5
Net investment income	3.2	2.5
Net investment income (expense) - BAM Surplus Note interest	(6.6)	(2.9)
Net realized and unrealized investment gains (losses)	9.1	(21.6)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
Commission revenues	0.0	0.0
Other revenues	0.8	0.7
Total revenues	7.8	(19.8)
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	0.9	0.4
Cost of sales	0.0	0.0
General and administrative expenses	16.2	15.6
Change in fair value of contingent consideration	0.0	0.0
Interest expense	0.0	0.0
Total expenses	17.1	16.0
Pre-tax income (loss) from continuing operations	(9.3)	(35.8)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Earned insurance premiums	255.1	194.4
Net investment income	10.6	1.6
Net investment income (expense) - BAM Surplus Note interest		0.0
Net realized and unrealized investment gains (losses)	24.5	(17.5)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha		0.0
Commission revenues		0.0
Other revenues	(2.7)	(2.8)
Total revenues	287.5	175.7
Loss and loss adjustment expenses	147.8	122.0
Acquisition expenses	59.8	49.9
Cost of sales		0.0
General and administrative expenses	35.2	21.0
Change in fair value of contingent consideration	(2.4)	2.1
Interest expense	5.0	3.8
Total expenses	245.4	198.8
Pre-tax income (loss) from continuing operations		(23.1)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Segment Reporting Information [Line Items]
Earned insurance premiums	249.9
Net investment income	8.4
Net investment income (expense) - BAM Surplus Note interest	0.0
Net realized and unrealized investment gains (losses)	24.5
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0
Commission revenues	0.0
Other revenues	(2.7)
Total revenues	280.1
Loss and loss adjustment expenses	147.6
Acquisition expenses	58.9
Cost of sales	0.0
General and administrative expenses	35.1
Change in fair value of contingent consideration	(2.4)
Interest expense	5.0
Total expenses	244.2
Pre-tax income (loss) from continuing operations	35.9
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Outrigger Re
Segment Reporting Information [Line Items]
Earned insurance premiums	5.2
Net investment income	2.2
Net investment income (expense) - BAM Surplus Note interest	0.0
Net realized and unrealized investment gains (losses)	0.0
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0
Commission revenues	0.0
Other revenues	0.0
Total revenues	7.4
Loss and loss adjustment expenses	0.2
Acquisition expenses	0.9
Cost of sales	0.0
General and administrative expenses	0.1
Change in fair value of contingent consideration	0.0
Interest expense	0.0
Total expenses	1.2
Pre-tax income (loss) from continuing operations	6.2
Asset Management (Kudu)
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	14.2	12.6
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	29.6	22.3
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0
Commission revenues	0.0	0.0
Other revenues	0.0	0.0
Total revenues	43.8	34.9
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	3.8	2.8
Change in fair value of contingent consideration	0.0	0.0
Interest expense	4.7	2.8
Total expenses	8.5	5.6
Pre-tax income (loss) from continuing operations	35.3	29.3
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0
Net investment income	7.0	1.8
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0
Net realized and unrealized investment gains (losses)	41.8	31.9
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	85.2	18.8
Commission revenues	3.3	2.9
Other revenues	30.6	25.7
Total revenues	167.9	81.1
Loss and loss adjustment expenses	0.0	0.0
Acquisition expenses	0.0	0.0
Cost of sales	13.9	21.4
General and administrative expenses	39.7	29.8
Change in fair value of contingent consideration	0.0	0.0
Interest expense	0.8	0.3
Total expenses	54.4	51.5
Pre-tax income (loss) from continuing operations	$ 113.5	$ 29.6