CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenues:
Net investment income	$ 19.6	$ 21.0	$ 42.5	$ 37.0
Net realized and unrealized investment gains			(3.8)	280.8
Total revenues	259.6	129.0	200.4	562.6
Expenses:
Total expenses	126.9	113.2	233.7	265.1
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	132.7	15.8	(33.3)	297.5
Income tax (expense) benefit	(24.1)	0.1	1.4	(10.1)
Net income (loss) from continuing operations	108.6	15.9	(31.9)	287.4
Net (loss) income from sale of discontinued operations, net of tax	(1.0)	0.0	(0.1)	0.7
Net income (loss)	107.6	15.9	(32.0)	288.1
Net loss attributable to non-controlling interests	7.8	4.6	18.6	16.8
Net income (loss) attributable to White Mountains’s common shareholders	115.4	20.5	(13.4)	304.9
HG Global-BAM
Revenues:
Earned insurance premiums	5.6	4.3	11.0	8.5
Net investment income	4.9	5.4	10.4	10.7
Net realized and unrealized investment gains	14.4	11.5	20.5	23.3
Advertising and commission revenues	0.0	0.0	0.0
Other revenue	1.2	0.4	1.7	1.0
Total revenues	26.1	21.6	43.6	43.5
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	2.1	1.4	3.8	2.7
Other underwriting expenses	0.2	0.1	0.2	0.2
Cost of sales	0.0	0.0	0.0
General and administrative expenses	12.5	12.3	27.2	26.6
Amortization of other intangible assets	0.0	0.0	0.0
Interest expense	0.0		0.0
Total expenses	14.8	13.8	31.2	29.5
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	11.3	7.8	12.4	14.0
NSM Holding Co LLC
Revenues:
Earned insurance premiums	0.0		0.0
Net investment income	0.0		0.0
Net realized and unrealized investment gains	0.0		0.0
Advertising and commission revenues	63.0	54.9	116.0	98.2
Other revenue	13.1	11.2	25.1	17.2
Total revenues	76.1	66.1	141.1	115.4
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0
Other underwriting expenses	0.0		0.0
Cost of sales	0.0		0.0
General and administrative expenses	48.5	32.8	88.1	58.1
Broker commission expenses	21.0	18.1	39.3	33.6
Change in fair value of contingent consideration earnout liabilities	(1.7)	6.3	(2.3)	7.6
Amortization of other intangible assets	6.3	4.1	11.1	9.1
Interest expense	5.7	4.1	10.0	7.8
Total expenses	79.8	65.4	146.2	116.2
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	(3.7)	0.7	(5.1)	(0.8)
Kudu investments
Revenues:
Earned insurance premiums	0.0		0.0
Net investment income	5.6	4.0	12.9	4.0
Net realized and unrealized investment gains	16.5	0.4	(8.3)	0.4
Advertising and commission revenues	0.0		0.0
Other revenue	0.0	0.0	0.1	0.0
Total revenues	22.1	4.4	4.7	4.4
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0		0.0
Other underwriting expenses	0.0		0.0
Cost of sales	0.0		0.0
General and administrative expenses	2.8	2.2	5.3	2.2
Amortization of other intangible assets	0.1	0.0	0.2	0.0
Interest expense	1.5	0.0	2.9	0.0
Total expenses	4.4	2.2	8.4	2.2
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	17.7		(3.7)	2.2
MediaAlpha
Revenues:
Advertising and commission revenues	0.0	0.0	0.0	48.8
Total revenues	0.0	0.0	0.0	48.8
Expenses:
Cost of sales	0.0	0.0	0.0	40.6
General and administrative expenses	0.0	0.0	0.0	12.5
Amortization of other intangible assets	0.0	0.0	0.0	1.6
Interest expense	0.0	0.0	0.0	0.2
Total expenses	0.0	0.0	0.0	54.9
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations				(6.1)
Other Segment
Revenues:
Earned insurance premiums	0.0	0.0	0.0
Net investment income	9.1	11.6	19.2	22.3
Net realized and unrealized investment gains	122.0	23.3	(16.0)	142.4
Realized gain and unrealized investment gain from the MediaAlpha Transaction		0.0	0.0	182.2
Advertising and commission revenues	1.9	1.3	4.0	2.6
Other revenue	2.3	0.7	3.8	1.0
Total revenues	135.3	36.9	11.0	350.5
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0	0.0	0.0
Other underwriting expenses	0.0		0.0
Cost of sales	2.2	1.6	4.2	2.7
General and administrative expenses	25.3	30.1	42.8	59.5
Amortization of other intangible assets	0.2	0.1	0.4	0.1
Interest expense	0.2	0.0	0.5	0.0
Total expenses	27.9	31.8	47.9	62.3
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	$ 107.4	$ 5.1	$ (36.9)	$ 288.2