CONSOLIDATED STATEMENTS OF OPERATIONS (Unaudited) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Revenues:
Net investment income	$ 18.4	$ 13.4	$ 55.4	$ 44.9
Net realized and unrealized investment gains (losses)			347.8	22.9
Total revenues	155.5	198.7	718.1	363.1
Expenses:
Total expenses	104.4	154.4	369.5	397.2
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	51.1	44.3	348.6	(34.1)
Income (expense) tax benefit	(8.8)	3.6	(18.9)	0.4
Net income (loss) from continuing operations	42.3	47.9	329.7	(33.7)
Net gain from sale of discontinued operations, net of tax	0.9	(17.3)	1.6	(17.2)
Net income (loss)	43.2	30.6	331.3	(50.9)
Net loss attributable to non-controlling interests	5.5	10.2	22.3	47.2
Net income (loss) attributable to White Mountains’s common shareholders	48.7	40.8	353.6	(3.7)
HG Global-BAM
Revenues:
Earned insurance premiums	5.2	3.3	13.7	9.7
Net investment income	5.4	4.8	16.1	12.5
Net realized and unrealized investment gains (losses)	7.1	(4.1)	30.4	(14.4)
Commission revenues	0.0	0.0	0.0	0.0
Other revenue	0.3	0.2	1.3	0.8
Total revenues	18.0	4.2	61.5	8.6
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	1.4	1.2	4.1	3.9
Other underwriting expenses	0.1	0.1	0.3	0.3
General and administrative expenses	12.5	11.3	39.1	36.5
Cost of sales	0.0	0.0	0.0	0.0
Amortization of other intangible assets	0.0	0.0	0.0	0.0
Interest expense	0.0	0.0	0.0	0.0
Total expenses	14.0	12.6	43.5	40.7
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	4.0	(8.4)	18.0	(32.1)
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 (losses)	0.0		0.0
Commission revenues	49.4	35.0	147.6	56.4
Other revenue	10.5	4.7	27.7	6.8
Total revenues	59.9	39.7	175.3	63.2
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0		0.0
Other underwriting expenses	0.0		0.0
General and administrative expenses	33.3	23.5	91.4	35.7
Broker commission expense	15.3	10.6	48.9	17.1
Change in fair value of contingent consideration earnout liabilities	(2.0)	2.6	5.6	2.6
Cost of sales	0.0		0.0
Amortization of other intangible assets	4.4	5.0	13.5	5.0
Interest expense	4.4	3.2	12.2	4.8
Total expenses	55.4	44.9	171.6	65.2
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	4.5	(5.2)	3.7	(2.0)
Kudu Investment Management, LLC
Revenues:
Earned insurance premiums	0.0
Net investment income	4.6	0.0	8.6	0.0
Net realized and unrealized investment gains (losses)	2.7	0.0	3.1	0.0
Commission revenues	0.0
Other revenue	0.2		0.2	0.0
Total revenues	7.5	0.0	11.9	0.0
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0
Other underwriting expenses	0.0
General and administrative expenses	4.5	0.0	6.7	0.0
Cost of sales	0.0
Amortization of other intangible assets	0.2	0.0	0.2	0.0
Total expenses	4.7	0.0	6.9	0.0
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	2.8		5.0
MediaAlpha
Revenues:
Earned insurance premiums		0.0	0.0	0.0
Net investment income		0.0	0.0	0.0
Net realized and unrealized investment gains (losses)		0.0	0.0	0.0
Commission revenues	0.0	74.5	48.8	216.4
Other revenue	0.0	0.0	0.0	1.6
Total revenues	0.0	74.5	48.8	218.0
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense		0.0	0.0	0.0
Other underwriting expenses		0.0	0.0	0.0
General and administrative expenses	0.0	5.4	12.5	21.2
Cost of sales	0.0	61.8	40.6	179.1
Amortization of other intangible assets	0.0	2.4	1.6	7.8
Interest expense	0.0	0.2	0.2	0.9
Total expenses	0.0	69.8	54.9	209.0
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations		4.7	(6.1)	9.0
Other Segment
Revenues:
Earned insurance premiums	0.0	0.0	0.0	0.0
Net investment income	8.4	8.6	30.7	32.4
Net realized and unrealized investment gains (losses)	57.2	70.2	199.6	37.3
Realized gain and unrealized investment gain from the MediaAlpha Transaction			182.2	0.0
Commission revenues	1.9	1.1	4.5	3.0
Other revenue	2.6	0.4	3.6	0.6
Total revenues	70.1	80.3	420.6	73.3
Expenses:
Deferred Policy Acquisition Costs, Amortization Expense	0.0		0.0	0.0
Other underwriting expenses	0.0		0.0	0.0
General and administrative expenses	27.7	26.0	87.2	79.0
Cost of sales	2.3	1.1	5.0	2.9
Amortization of other intangible assets	0.0	0.0	0.1	0.1
Interest expense	0.3	0.0	0.3	0.3
Total expenses	30.3	27.1	92.6	82.3
Pre-Tax and Net Income [Abstract]
Pre-tax income (loss) from continuing operations	39.8	53.2	328.0	(9.0)
Discontinued Operations
Pre-Tax and Net Income [Abstract]
Net income (loss) attributable to White Mountains’s common shareholders	$ 48.7	$ 40.8	353.6	(3.7)
Swedish Tax Authority | Discontinued Operations | Sirius Group
Pre-Tax and Net Income [Abstract]
Net gain from sale of discontinued operations, net of tax				$ 0.1
MediaAlpha | HG Global-BAM
Revenues:
Realized gain and unrealized investment gain from the MediaAlpha Transaction			0.0
MediaAlpha | NSM Holding Co LLC
Revenues:
Realized gain and unrealized investment gain from the MediaAlpha Transaction			$ 0.0