Segment Information - Financial Information for White Mountain's Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Segment Reporting Information [Line Items]
Earned insurance premiums	$ 301.0	$ 227.8	$ 563.8	$ 430.6
Net investment income	43.0	25.1	82.0	45.7
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	88.7	(104.1)	201.6	(112.5)
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	(77.3)	(113.5)	7.9	(94.7)
Commission revenues	3.2	2.6	6.5	5.5
Other revenues	19.8	38.8	48.5	62.5
Total revenues	378.4	76.7	910.3	337.1
Loss and loss adjustment expenses	167.5	120.5	315.3	242.5
Acquisition expenses	62.9	55.0	125.4	107.9
Cost of sales	11.6	22.4	25.5	43.8
General and administrative expenses	102.6	101.2	198.6	171.1
Change in fair value of contingent consideration	2.2	0.1	(0.2)	2.2
Interest expense	14.2	10.1	29.2	17.0
Total expenses	361.0	309.3	693.8	584.5
Pre-tax income (loss) from continuing operations	17.4	(232.6)	216.5	(247.4)
Financial Guarantee (HG Global/BAM)
Segment Reporting Information [Line Items]
Earned insurance premiums	7.7	10.5	15.4	18.9
Net investment income	7.6	4.8	14.8	9.4
Net realized and unrealized investment gains (losses)	(9.9)	(30.1)	7.1	(75.2)
Other revenues	0.5	1.6	1.3	2.4
Total revenues	5.9	(13.2)	38.6	(44.5)
Acquisition expenses	1.5	4.8	4.2	7.8
General and administrative expenses	14.9	17.4	32.2	33.7
Interest expense	2.5	3.4	7.0	3.4
Total expenses	18.9	25.6	43.4	44.9
Financial Guarantee (HG Global/BAM) | HG Global
Segment Reporting Information [Line Items]
Earned insurance premiums	6.4	8.7	12.8	15.6
Net investment income	4.1	2.2	8.1	4.3
Net investment income (expense) - BAM Surplus Note interest	6.5	3.0	13.1	5.9
Net realized and unrealized investment gains (losses)	(5.7)	(14.7)	2.2	(38.2)
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	11.3	(0.7)	36.2	(12.2)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	1.8	3.4	3.6	6.0
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	0.3	0.8	1.4	1.5
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	2.5	3.4	7.0	3.4
Total expenses	4.6	7.6	12.0	10.9
Pre-tax income (loss) from continuing operations	6.7	(8.3)	24.2	(23.1)
Financial Guarantee (HG Global/BAM) | BAM
Segment Reporting Information [Line Items]
Earned insurance premiums	1.3	1.8	2.6	3.3
Net investment income	3.5	2.6	6.7	5.1
Net investment income (expense) - BAM Surplus Note interest	(6.5)	(3.0)	(13.1)	(5.9)
Net realized and unrealized investment gains (losses)	(4.2)	(15.4)	4.9	(37.0)
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	0.5	1.5	1.3	2.2
Total revenues	(5.4)	(12.5)	2.4	(32.3)
Loss and loss adjustment expenses	0.0	0.0	0.0	0.0
Acquisition expenses	(0.3)	1.4	0.6	1.8
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	14.6	16.6	30.8	32.2
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	14.3	18.0	31.4	34.0
Pre-tax income (loss) from continuing operations	(19.7)	(30.5)	(29.0)	(66.3)
P&C Insurance and Reinsurance (Ark/WM Outrigger)
Segment Reporting Information [Line Items]
Earned insurance premiums	293.3	217.3	548.4	411.7
Net investment income	13.7	3.2	24.3	4.8
Net realized and unrealized investment gains (losses)	18.0	(44.6)	42.5	(62.1)
Other revenues	(2.0)	6.3	(4.7)	3.5
Total revenues	323.0	182.2	610.5	357.9
Loss and loss adjustment expenses	167.5	120.5	315.3	242.5
Acquisition expenses	61.4	50.2	121.2	100.1
General and administrative expenses	34.9	29.7	70.1	50.7
Change in fair value of contingent consideration	2.2	0.1	(0.2)	2.2
Interest expense	5.2	3.1	10.2	6.9
Total expenses	271.2	203.6	516.6	402.4
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Ark
Segment Reporting Information [Line Items]
Earned insurance premiums	283.7	217.3	533.6	411.7
Net investment income	11.2	3.2	19.6	4.8
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	18.0	(44.6)	42.5	(62.1)
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	(2.0)	6.3	(4.7)	3.5
Total revenues	310.9	182.2	591.0	357.9
Loss and loss adjustment expenses	167.1	120.5	314.7	242.5
Acquisition expenses	59.4	50.2	118.3	100.1
Cost of sales	0.0	0.0	0.0	0.0
General and administrative expenses	34.9	29.7	70.0	50.7
Change in fair value of contingent consideration	2.2	0.1	(0.2)	2.2
Interest expense	5.2	3.1	10.2	6.9
Total expenses	268.8	203.6	513.0	402.4
Pre-tax income (loss) from continuing operations	42.1	(21.4)	78.0	(44.5)
P&C Insurance and Reinsurance (Ark/WM Outrigger) | Outrigger Re
Segment Reporting Information [Line Items]
Earned insurance premiums	9.6		14.8
Net investment income	2.5		4.7
Net investment income (expense) - BAM Surplus Note interest	0.0		0.0
Net realized and unrealized investment gains (losses)	0.0		0.0
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	12.1		19.5
Loss and loss adjustment expenses	0.4		0.6
Acquisition expenses	2.0		2.9
Cost of sales	0.0		0.0
General and administrative expenses	0.0		0.1
Change in fair value of contingent consideration	0.0		0.0
Interest expense	0.0		0.0
Total expenses	2.4		3.6
Pre-tax income (loss) from continuing operations	9.7		15.9
Asset Management (Kudu)
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	14.7	13.8	28.9	26.4
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	4.6	(17.6)	34.2	4.7
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	0.0
Total revenues	19.3	(3.8)	63.1	31.1
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	4.0	3.1	7.8	5.9
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	5.3	3.3	10.0	6.1
Total expenses	9.3	6.4	17.8	12.0
Pre-tax income (loss) from continuing operations	10.0	(10.2)	45.3	19.1
Other Operations
Segment Reporting Information [Line Items]
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	7.0	3.3	14.0	5.1
Net investment income (expense) - BAM Surplus Note interest	0.0	0.0	0.0	0.0
Net realized and unrealized investment gains (losses)	76.0	(11.8)	117.8	20.1
Net realized and unrealized investment gains (losses) from investment in MediaAlpha	(77.3)	(113.5)	7.9	(94.7)
Commission revenues	3.2	2.6	6.5	5.5
Other revenues	21.3	30.9	51.9	56.6
Total revenues	30.2	(88.5)	198.1	(7.4)
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	11.6	22.4	25.5	43.8
General and administrative expenses	48.8	51.0	88.5	80.8
Change in fair value of contingent consideration	0.0	0.0	0.0	0.0
Interest expense	1.2	0.3	2.0	0.6
Total expenses	61.6	73.7	116.0	125.2
Pre-tax income (loss) from continuing operations	$ (31.4)	$ (162.2)	$ 82.1	$ (132.6)