Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 227.8	$ 124.3	$ 430.6	$ 235.3
Net investment income	25.1	17.1	45.7	37.7
Net realized and unrealized investment gains (losses)	(104.1)	59.6	(112.5)	60.7
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	(113.5)	113.0	(94.7)	71.3
Commission revenues	2.6	2.3	5.5	4.6
Other revenues	38.8	26.1	62.5	36.2
Total revenues	76.7	342.4	337.1	445.8
Loss and loss adjustment expenses	120.5	52.6	242.5	118.6
Insurance and reinsurance acquisition expenses	55.0	35.6	107.9	74.2
Cost of sales	22.4	17.9	43.8	21.9
General and administrative expenses	100.3	71.2	171.3	163.6
Amortization of other intangible assets	1.0	0.5	2.0	1.1
Interest expense	10.1	3.2	17.0	10.4
Total expenses	309.3	181.0	584.5	389.8
Pre-tax income (loss) from continuing operations	(232.6)	161.4	(247.4)	56.0
Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Earned insurance premiums	10.5	6.5	18.9	12.9
Net investment income	4.8	4.3	9.4	8.8
Net realized and unrealized investment gains (losses)	(30.1)	6.3	(75.2)	(11.6)
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	1.6	0.3	2.4	0.6
Total revenues	(13.2)	17.4	(44.5)	10.7
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Insurance and reinsurance acquisition expenses	4.8	1.6	7.8	3.5
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	17.4	13.9	33.7	30.3
Amortization of other intangible assets	0.0	0.0	0.0	0.0
Interest expense	3.4	0.0	3.4	0.0
Total expenses	25.6	15.5	44.9	33.8
Pre-tax income (loss) from continuing operations	(38.8)	1.9	(89.4)	(23.1)
Ark
Segment Reporting Information [Line Items]
Earned insurance premiums	217.3	117.8	411.7	222.4
Net investment income	3.2	0.4	4.8	1.2
Net realized and unrealized investment gains (losses)	(44.6)	8.9	(62.1)	10.0
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	6.3	3.4	3.5	6.0
Total revenues	182.2	130.5	357.9	239.6
Loss and loss adjustment expenses	120.5	52.6	242.5	118.6
Insurance and reinsurance acquisition expenses	50.2	34.0	100.1	70.7
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	29.8	24.8	52.9	62.6
Amortization of other intangible assets	0.0	0.0	0.0	0.0
Interest expense	3.1	1.3	6.9	2.4
Total expenses	203.6	112.7	402.4	254.3
Pre-tax income (loss) from continuing operations	(21.4)	17.8	(44.5)	(14.7)
Asset Management (Kudu)
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	13.8	8.4	26.4	16.6
Net realized and unrealized investment gains (losses)	(17.6)	27.8	4.7	43.6
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	0.0	0.0	0.0	0.1
Total revenues	(3.8)	36.2	31.1	60.3
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	3.0	3.2	5.7	5.7
Amortization of other intangible assets	0.1	0.1	0.2	0.2
Interest expense	3.3	1.5	6.1	7.3
Total expenses	6.4	4.8	12.0	13.2
Pre-tax income (loss) from continuing operations	(10.2)	31.4	19.1	47.1
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	3.3	4.0	5.1	11.1
Net realized and unrealized investment gains (losses)	(11.8)	16.6	20.1	18.7
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	(113.5)	113.0	(94.7)	71.3
Commission revenues	2.6	2.3	5.5	4.6
Other revenues	30.9	22.4	56.6	29.5
Total revenues	(88.5)	158.3	(7.4)	135.2
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	22.4	17.9	43.8	21.9
General and administrative expenses	50.1	29.3	79.0	65.0
Amortization of other intangible assets	0.9	0.4	1.8	0.9
Interest expense	0.3	0.4	0.6	0.7
Total expenses	73.7	48.0	125.2	88.5
Pre-tax income (loss) from continuing operations	$ (162.2)	$ 110.3	$ (132.6)	$ 46.7