CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) $ in Millions		3 Months Ended									9 Months Ended	12 Months Ended
	Sep. 28, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 28, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Earned insurance premiums												$ 13.9	$ 10.4	$ 13.4
Net investment income												59.0	56.0	32.1
Net realized and unrealized investment gains												(108.3)	133.3	(27.4)
Commission revenues												394.3	167.0	118.3
Other revenue												10.2	7.1	21.3
Total revenues		$ 6.0	$ 198.7	$ 122.3	$ 42.1	$ 114.0	$ 87.5	$ 83.5	$ 88.8			369.1	373.8	157.7
Expenses
Loss and loss adjustment expenses													1.1	8.0
Insurance acquisition expenses												5.3	4.1	5.6
Other underwriting expenses												0.4	0.4	0.4
Cost of sales												248.7	139.4	102.0
General and administrative expenses												235.7	208.0	175.5
Broker commission expense												28.9
Amortization of other intangible assets												18.8	10.7	10.5
Interest expense on debt												9.5	2.3	3.0
Total expenses		150.1	154.4	134.7	108.1	108.8	79.1	85.7	92.4			547.3	366.0	305.0
Pre-tax income (loss)		(144.1)	44.3	(12.4)	(66.0)	5.2	8.4	(2.2)	(3.6)			(178.2)	7.8	(147.3)
Income tax (expense) benefit		3.6	3.6	(2.5)	(0.7)	2.5	4.0	1.0	0.3			4.0	7.8	32.9
Net income (loss) from continuing operations		(140.5)	47.9	(14.9)	(66.7)	7.7	12.4	(1.2)	(3.3)			(174.2)	15.6	(114.4)
Gain (loss) on sale of other discontinued operations, net of tax												(17.2)	557.0	415.1
Net income from discontinued operations, net of tax		0.0	(17.3)	0.0	0.1	4.3	539.1	2.8	31.3			0.0	20.5	108.3
Net income												(191.4)	593.1	409.0
Net loss attributable to non-controlling interests		3.0	10.2	18.4	18.6	10.5	10.6	12.0	1.0			50.2	34.1	(7.2)
Net income attributable to White Mountains’s common shareholders		$ (137.5)	$ 40.8	$ 3.5	$ (48.0)	$ 22.5	$ 562.1	$ 13.6	$ 29.0			(141.2)	627.2	401.8
HG Global-BAM
Revenues
Earned insurance premiums												13.9	9.4	5.9
Net investment income												16.7	12.3	9.0
Net realized and unrealized investment gains												(7.5)	0.6	0.7
Commission revenues												0.0	0.0	0.0
Other revenue												1.2	1.0	1.1
Total revenues												24.3	23.3	16.7
Expenses
Loss and loss adjustment expenses													0.0	0.0
Insurance acquisition expenses												5.3	4.0	3.4
Other underwriting expenses												0.4	0.4	0.4
Cost of sales												0.0	0.0	0.0
General and administrative expenses												48.0	42.9	39.6
Broker commission expense												0.0
Amortization of other intangible assets												0.0	0.0	0.0
Interest expense on debt												0.0	0.0	0.0
Total expenses												53.7	47.3	43.4
Pre-tax income (loss)												(29.4)	(24.0)	(26.7)
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												94.7	0.0	0.0
Other revenue												6.9	0.0	0.0
Total revenues												101.6	0.0	0.0
Expenses
Insurance acquisition expenses												0.0
Other underwriting expenses												0.0
Cost of sales												0.0
General and administrative expenses												61.6	0.0	0.0
Broker commission expense												28.9	0.0	0.0
Amortization of other intangible assets												8.3	0.0	0.0
Interest expense on debt												8.0	0.0	0.0
Total expenses												106.8	0.0	0.0
Pre-tax income (loss)												(5.2)
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												0.0	0.0	0.0
Commission revenues												295.5	163.2	116.5
Other revenue												1.6	0.0	0.0
Total revenues												297.1	163.2	116.5
Expenses
Loss and loss adjustment expenses													0.0	0.0
Insurance acquisition expenses												0.0	0.0	0.0
Other underwriting expenses												0.0	0.0	0.0
Cost of sales												245.0	135.9	97.8
General and administrative expenses												31.7	16.2	11.8
Broker commission expense												0.0
Amortization of other intangible assets												10.3	10.5	10.1
Interest expense on debt												1.2	1.0	0.9
Total expenses												288.2	163.6	120.6
Pre-tax income (loss)												8.9	(0.4)	(4.1)
Other
Revenues
Earned insurance premiums												0.0	1.0	7.5
Net investment income												42.3	43.7	23.1
Net realized and unrealized investment gains												(100.8)	132.7	(28.1)
Commission revenues												4.1	3.8	1.8
Other revenue												0.5	6.1	20.2
Total revenues												(53.9)	187.3	24.5
Expenses
Loss and loss adjustment expenses												0.0	1.1	8.0
Insurance acquisition expenses												0.0	0.1	2.2
Other underwriting expenses												0.0	0.0	0.0
Cost of sales												3.7	3.5	4.2
General and administrative expenses												94.4	148.9	124.1
Broker commission expense												0.0
Amortization of other intangible assets												0.2	0.2	0.4
Interest expense on debt												0.3	1.3	2.1
Total expenses												98.6	155.1	141.0
Pre-tax income (loss)												(152.5)	32.2	(116.5)
OneBeacon
Expenses
Gain (loss) on sale of other discontinued operations, net of tax	$ 554.6
Net income from discontinued operations, net of tax											$ 20.5			108.6
Tranzact
Expenses
Gain (loss) on sale of other discontinued operations, net of tax													3.2	51.9
Sirius Group
Revenues
Net realized and unrealized investment gains														3.7
Expenses
Gain (loss) on sale of other discontinued operations, net of tax										$ (4.0)				363.2
Discontinued Operations
Expenses
Net income attributable to White Mountains’s common shareholders												(141.2)	627.2	401.8
Discontinued Operations | OneBeacon
Revenues
Earned insurance premiums													807.6	1,100.6
Net investment income													39.7	50.6
Net realized and unrealized investment gains													38.8	37.7
Other revenue													7.7	5.5
Total revenues													893.8	1,194.4
Expenses
Loss and loss adjustment expenses													546.0	656.0
Other underwriting expenses													156.2	209.0
General and administrative expenses													21.2	14.2
Interest expense on debt													10.0	13.1
Total expenses													879.0	1,098.3
Gain (loss) on sale of other discontinued operations, net of tax												0.0	554.5	0.0
Net income from discontinued operations, net of tax													20.5	108.6
Discontinued Operations | Tranzact
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
Other revenue													0.0	119.6
Total revenues													0.0	119.6
Expenses
Loss and loss adjustment expenses													0.0	0.0
Other underwriting expenses													0.0	0.0
General and administrative expenses													0.0	116.7
Interest expense on debt														3.2
Total expenses													0.0	119.9
Income tax (expense) benefit														21.4
Gain (loss) on sale of other discontinued operations, net of tax												0.0	3.2	51.9
Net income from discontinued operations, net of tax													0.0	6.1
Discontinued Operations | Sirius Group
Revenues
Earned insurance premiums													0.0	240.1
Net investment income													0.0	14.4
Net realized and unrealized investment gains													0.0	(1.5)
Other revenue													0.0	0.6
Total revenues													0.0	253.6
Expenses
Loss and loss adjustment expenses													0.0	154.9
Other underwriting expenses													0.0	30.9
General and administrative expenses													0.0	10.4
Interest expense on debt														7.9
Total expenses													0.0	263.1
Gain (loss) on sale of other discontinued operations, net of tax												$ (17.2)	(0.7)	363.2
Net income from discontinued operations, net of tax													$ 0.0	$ (6.4)