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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - Statement of Operations and Comprehensive Income (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 944.8	$ 1,180.7	$ 893.4
Expenses	1,246.8	535.5	488.3
Pre-tax income (loss)	(302.0)	645.2	405.1
Income Tax Expense (Benefit)	(38.6)	20.5	(29.3)
Net income (loss) from continuing operations	(340.6)	665.7	375.8
Net income (loss) attributable to non-controlling interests	46.5	45.3	37.9
Net income (loss) attributable to White Mountains’s common shareholders	(275.4)	708.7	414.5
Other comprehensive income (loss), net of tax	1.9	7.3	(1.4)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	(273.3)	715.5	413.1
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	(0.2)	(8.7)	65.0
Expenses	39.0	61.0	47.1
Pre-tax income (loss)	(39.2)	(69.7)	17.9
Income Tax Expense (Benefit)	0.0	(0.3)	(0.9)
Net income (loss) from continuing operations	(39.2)	(70.0)	17.0
Net income from discontinued operations, net of tax	18.7	(2.3)	0.8
Equity in earnings from consolidated and unconsolidated subsidiaries, net of tax	(257.4)	782.0	398.5
Net income (loss) attributable to non-controlling interests	2.5	(1.0)	(1.8)
Net income (loss) attributable to White Mountains’s common shareholders	(275.4)	708.7	414.5
Other comprehensive income (loss), net of tax	2.1	6.8	(1.4)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	(273.3)	715.5	413.1
Swedish Tax Authority | Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Loss contingency accrual, period increase (decrease)	18.7	(2.3)	0.8
Swedish Tax Authority | Sirius Group | White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Loss contingency accrual, period increase (decrease)	$ 18.7	$ (2.3)	$ 0.8

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