SCHEDULE II - CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT - CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 2,166.7	$ 1,157.9	$ 614.4
Expenses	1,601.3	1,307.3	888.0
Pre-tax income (loss) from continuing operations	565.4	(149.4)	(273.6)
Income tax (expense) benefit	15.5	(41.4)	(44.4)
Net income (loss) from continuing operations	580.9	(190.8)	(318.0)
Net income (loss) from discontinued operations, net of tax - NSM Group		902.5	(2.9)
Net income (loss) attributable to White Mountains’s common shareholders	509.2	792.8	(275.4)
Other comprehensive income (loss), net of tax	2.4	(3.8)	1.7
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	(2.9)	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	511.1	787.6	(273.3)
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	81.7	(0.4)	(0.2)
Expenses	60.5	67.1	39.0
Pre-tax income (loss) from continuing operations	21.2	(67.5)	(39.2)
Income tax (expense) benefit	(1.2)	(0.9)	0.0
Net income (loss) from continuing operations	20.0	(68.4)	(39.2)
Equity in (earnings) losses from consolidated and unconsolidated subsidiaries, net of tax	489.2	(42.0)	(232.3)
Net income (loss) from discontinued operations, net of tax - NSM Group	0.0	16.4	(22.6)
Net income (loss) attributable to White Mountains’s common shareholders	509.2	792.8	(275.4)
Other comprehensive income (loss), net of tax	1.9	(2.9)	1.9
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	(5.2)	0.2
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	2.9	0.0
Comprehensive income (loss) attributable to White Mountains’s common shareholders	511.1	787.6	(273.3)
Sirius Group
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	0.0	18.7
Loss contingency accrual, period increase (decrease)			18.7
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	0.0	18.7
NSM
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	886.8	0.0
NSM | 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	$ 886.8	$ 0.0