CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Revenues:
Net investment income	$ 22.9	$ 16.0
Net realized and unrealized investment gains	(156.7)	245.6
Total revenues	(59.2)	433.6
Expenses:
Total expenses	106.8	151.9
Pre-Tax and Net Income [Abstract]
Pre-tax (loss) income from continuing operations	(166.0)	281.7
Income tax benefit (expense)	25.5	(10.2)
Net (loss) income from continuing operations	(140.5)	271.5
Net gain from sale of discontinued operations, net of tax	0.9	0.7
Net income (loss)	(139.6)	272.2
Net loss attributable to non-controlling interests	10.8	12.2
Net (loss) income attributable to White Mountains’s common shareholders	(128.8)	284.4
HG Global-BAM
Revenues:
Earned insurance premiums	5.4	4.2
Net investment income	5.5	5.3
Net realized and unrealized investment gains	6.1	11.8
Commission revenues	0.0	0.0
Other revenue	0.5	0.6
Total revenues	17.5	21.9
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	1.7	1.3
Cost of sales		0.0
General and administrative expenses	14.7	14.4
Cost of sales	0.0
Amortization of other intangible assets	0.0	0.0
Interest expense	0.0
Total expenses	16.4	15.7
Pre-Tax and Net Income [Abstract]
Pre-tax (loss) income from continuing operations	1.1	6.2
NSM Holding Co LLC
Revenues:
Earned insurance premiums	0.0
Net investment income	0.0
Net realized and unrealized investment gains	0.0
Commission revenues	53.0	43.3
Other revenue	12.0	6.0
Total revenues	65.0	49.3
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0
General and administrative expenses	39.6	25.3
Broker commission expenses	18.3	15.5
Change in fair value of contingent consideration earnout liabilities	(0.6)	1.3
Cost of sales	0.0
Amortization of other intangible assets	4.8	5.0
Interest expense	4.3	3.7
Total expenses	66.4	50.8
Pre-Tax and Net Income [Abstract]
Pre-tax (loss) income from continuing operations	(1.4)	(1.5)
Kudu investments
Revenues:
Earned insurance premiums	0.0
Net investment income	7.3	0.0
Net realized and unrealized investment gains	(24.8)	0.0
Commission revenues	0.0
Other revenue	0.1
Total revenues	(17.4)	0.0
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0
General and administrative expenses	2.5	0.0
Cost of sales	0.0
Amortization of other intangible assets	0.1	0.0
Interest expense	1.4	0.0
Total expenses	4.0	0.0
Pre-Tax and Net Income [Abstract]
Pre-tax (loss) income from continuing operations	(21.4)
MediaAlpha
Revenues:
Earned insurance premiums		0.0
Net investment income		0.0
Net realized and unrealized investment gains		0.0
Commission revenues	0.0	48.8
Other revenue	0.0	0.0
Total revenues	0.0	48.8
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense		0.0
General and administrative expenses	0.0	12.5
Cost of sales	0.0	40.6
Amortization of other intangible assets	0.0	1.6
Interest expense	0.0	0.2
Total expenses	0.0	54.9
Pre-Tax and Net Income [Abstract]
Pre-tax (loss) income from continuing operations		(6.1)
Other Segment
Revenues:
Earned insurance premiums	0.0	0.0
Net investment income	10.1	10.7
Net realized and unrealized investment gains	(138.0)	119.1
Realized gain and unrealized investment gain from the MediaAlpha Transaction		182.2
Commission revenues	2.1	1.3
Other revenue	1.5	0.3
Total revenues	(124.3)	313.6
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0	0.0
General and administrative expenses	17.5	29.4
Cost of sales	2.0	1.1
Amortization of other intangible assets	0.2	0.0
Interest expense	0.3	0.0
Total expenses	20.0	30.5
Pre-Tax and Net Income [Abstract]
Pre-tax (loss) income from continuing operations	(144.3)	283.1
Discontinued Operations
Pre-Tax and Net Income [Abstract]
Net (loss) income attributable to White Mountains’s common shareholders	$ (128.8)	$ 284.4