Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 353.2	$ 220.1	$ 783.8	$ 455.4
Net investment income	33.9	19.5	79.6	57.2
Net realized and unrealized investment gains (losses)	(29.4)	30.5	(141.9)	91.2
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	(18.6)	(396.8)	(113.3)	(325.5)
Commission revenues	3.2	2.4	8.7	7.0
Other revenues	40.9	31.9	103.4	68.1
Total revenues	383.2	(92.4)	720.3	353.4
Loss and loss adjustment expenses	213.7	129.2	456.2	247.8
Insurance and reinsurance acquisition expenses	76.5	56.7	184.4	130.9
Cost of sales	25.0	24.0	68.8	45.9
General and administrative expenses	87.1	51.9	258.4	215.5
Amortization of other intangible assets	1.4	2.0	3.4	3.1
Interest expense	10.5	4.4	27.5	14.8
Total expenses	414.2	268.2	998.7	658.0
Pre-tax income (loss) from continuing operations	(31.0)	(360.6)	(278.4)	(304.6)
Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Earned insurance premiums	7.1	6.7	26.0	19.6
Net investment income	5.7	4.4	15.1	13.2
Net realized and unrealized investment gains (losses)	(38.8)	(4.0)	(114.0)	(15.6)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0	0.0	0.0
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	1.3	0.3	3.7	0.9
Total revenues	(24.7)	7.4	(69.2)	18.1
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and reinsurance acquisition expenses	1.7	3.0	9.5	6.5
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	15.8	12.4	49.5	42.7
Amortization of other intangible assets	0.0	0.0	0.0	0.0
Interest expense	2.0	0.0	5.4	0.0
Total expenses	19.5	15.4	64.4	49.2
Pre-tax income (loss) from continuing operations	(44.2)	(8.0)	(133.6)	(31.1)
Ark
Segment Reporting Information [Line Items]
Earned insurance premiums	346.1	213.4	757.8	435.8
Net investment income	4.9	0.6	9.7	1.8
Net realized and unrealized investment gains (losses)	(14.4)	0.3	(76.5)	10.3
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0	0.0	0.0
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	6.6	3.4	10.1	9.4
Total revenues	343.2	217.7	701.1	457.3
Loss and loss adjustment expenses	213.7	129.2	456.2	247.8
Insurance and reinsurance acquisition expenses	74.8	53.7	174.9	124.4
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	26.9	21.8	79.8	84.4
Amortization of other intangible assets	0.0	0.0	0.0	0.0
Interest expense	3.7	2.1	10.6	4.5
Total expenses	319.1	206.8	721.5	461.1
Pre-tax income (loss) from continuing operations	24.1	10.9	(20.4)	(3.8)
Asset Management (Kudu)
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	14.8	9.5	41.2	26.1
Net realized and unrealized investment gains (losses)	41.1	18.9	45.8	62.5
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	0.0	0.0	0.0	0.0
Commission revenues	0.0	0.0	0.0	0.0
Other revenues	0.0	0.1	0.0	0.2
Total revenues	55.9	28.5	87.0	88.8
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and reinsurance 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	3.3	10.2	9.0
Amortization of other intangible assets	0.0	0.0	0.2	0.2
Interest expense	4.2	1.9	10.3	9.2
Total expenses	8.7	5.2	20.7	18.4
Pre-tax income (loss) from continuing operations	47.2	23.3	66.3	70.4
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	8.5	5.0	13.6	16.1
Net realized and unrealized investment gains (losses)	(17.3)	15.3	2.8	34.0
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	(18.6)	(396.8)	(113.3)	(325.5)
Commission revenues	3.2	2.4	8.7	7.0
Other revenues	33.0	28.1	89.6	57.6
Total revenues	8.8	(346.0)	1.4	(210.8)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and reinsurance acquisition expenses	0.0	0.0	0.0	0.0
Cost of sales	25.0	24.0	68.8	45.9
General and administrative expenses	39.9	14.4	118.9	79.4
Amortization of other intangible assets	1.4	2.0	3.2	2.9
Interest expense	0.6	0.4	1.2	1.1
Total expenses	66.9	40.8	192.1	129.3
Pre-tax income (loss) from continuing operations	$ (58.1)	$ (386.8)	$ (190.7)	$ (340.1)