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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Operations and Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								9 Months Ended	12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 175.3	$ 155.5	$ 129.0	$ 433.6	$ 6.0	$ 198.7	$ 122.3	$ 42.1		$ 893.4	$ 369.1	$ 373.8
Expenses	118.8	104.4	113.2	151.9	150.1	154.4	134.7	108.1		488.3	547.3	366.0
Pre-tax income (loss)	56.5	51.1	15.8	281.7	(144.1)	44.3	(12.4)	(66.0)		405.1	(178.2)	7.8
Income tax (expense) benefit	(10.4)	(8.8)	0.1	(10.2)	3.6	3.6	(2.5)	(0.7)		(29.3)	4.0	7.8
Net income (loss) from continuing operations	46.1	42.3	15.9	271.5	(140.5)	47.9	(14.9)	(66.7)		375.8	(174.2)	15.6
Net income from discontinued operations, net of tax	(0.8)	0.9	0.0	0.7	0.0	(17.3)	0.0	0.1		0.0	0.0	20.5
Net loss attributable to non-controlling interests	15.6	5.5	4.6	12.2	3.0	10.2	18.4	18.6		37.9	50.2	34.1
Net income (loss) attributable to White Mountains’s common shareholders	$ 60.9	$ 48.7	$ 20.5	$ 284.4	$ (137.5)	$ 40.8	$ 3.5	$ (48.0)		414.5	(141.2)	627.2
Other comprehensive (loss) income, net of tax										(1.4)	(4.8)	0.3
Comprehensive income (loss) attributable to White Mountains’s common shareholders										413.1	(145.7)	630.5
Total income (loss) from discontinued operations										0.8	(17.2)	577.5
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)										65.0	(47.7)	27.3
Expenses										47.1	45.9	99.7
Pre-tax income (loss)										17.9	(93.6)	(72.4)
Income tax (expense) benefit										0.9	2.5	1.4
Net income (loss) from continuing operations										17.0	(96.1)	(73.8)
Net income from discontinued operations, net of tax										0.8	(17.2)	0.0
Equity in earnings of unconsolidated affiliates										398.5	(27.4)	701.0
Net loss attributable to non-controlling interests										(1.8)	(0.5)	0.0
Net income (loss) attributable to White Mountains’s common shareholders										414.5	(141.2)	627.2
Other comprehensive (loss) income, net of tax										(1.4)	(4.5)	3.3
Comprehensive income (loss) attributable to White Mountains’s common shareholders										413.1	(145.7)	630.5
Discontinued Operations
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income (loss) attributable to White Mountains’s common shareholders										414.5	(141.2)	627.2
Total income (loss) from discontinued operations										0.8	(17.2)	577.5
Discontinued Operations | Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)												0.0
Expenses												0.0
Net income from discontinued operations, net of tax												0.0
Total income (loss) from discontinued operations												(0.7)
Discontinued Operations | OneBeacon, Sirius Group, and Tranzact
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Total income (loss) from discontinued operations												$ 577.5
Swedish Tax Authority | Sirius Group | Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net income from discontinued operations, net of tax									$ (17.3)	0.8
Loss contingency accrual, period increase (decrease)										$ (0.8)	$ 17.3

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