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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, 2022	Dec. 31, 2021	Dec. 31, 2020
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 1,157.9	$ 614.4	$ 895.6
Expenses	1,307.3	888.0	235.2
Pre-tax income (loss) from continuing operations	(149.4)	(273.6)	660.4
Income tax (expense) benefit	(41.4)	(44.4)	14.8
Net income (loss) from continuing operations	(190.8)	(318.0)	675.2
Net (income) loss attributable to non-controlling interests	80.4	46.5	45.3
Net income (loss) attributable to White Mountains’s common shareholders	792.8	(275.4)	708.7
Other comprehensive income (loss), net of tax	(3.8)	1.7	1.4
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	(5.2)	0.2	5.9
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	0.0	0.0
Comprehensive income (loss)	786.7	(273.5)	716.0
Comprehensive (income) loss attributable to non-controlling interests	0.9	0.2	(0.5)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	787.6	(273.3)	715.5
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	(0.4)	(0.2)	(8.7)
Expenses	67.1	39.0	61.0
Pre-tax income (loss) from continuing operations	(67.5)	(39.2)	(69.7)
Income tax (expense) benefit	(0.9)	0.0	(0.3)
Net income (loss) from continuing operations	(68.4)	(39.2)	(70.0)
Equity in earnings from consolidated subsidiaries, net of tax	853.8	(257.4)	782.0
Net (income) loss attributable to non-controlling interests	7.4	2.5	(1.0)
Net income (loss) attributable to White Mountains’s common shareholders	792.8	(275.4)	708.7
Other comprehensive income (loss), net of tax	(3.8)	1.7	1.4
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	(5.2)	0.2	5.9
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	2.9	0.0	0.0
Comprehensive income (loss)	786.7	(273.5)	716.0
Comprehensive (income) loss attributable to non-controlling interests	0.9	0.2	(0.5)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	787.6	(273.3)	715.5
Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	18.7	(2.3)
Sirius Group | White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	$ 0.0	18.7	(2.3)
Swedish Tax Authority | Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Loss contingency accrual, period increase (decrease)		18.7	(2.3)
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)

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