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, 2024	Dec. 31, 2023	Dec. 31, 2022
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	$ 2,239.8	$ 2,166.7	$ 1,157.9
Expenses	1,923.1	1,601.3	1,307.3
Pre-tax income (loss) from continuing operations	316.7	565.4	(149.4)
Income tax (expense) benefit	(32.6)	15.5	(41.4)
Net income (loss) from continuing operations	284.1	580.9	(190.8)
Equity in earnings from discontinued operations, net of tax - NSM Group			902.5
Net income (loss) attributable to White Mountains’s common shareholders	230.4	509.2	792.8
Other comprehensive income (loss), net of tax	(0.1)	2.4	(3.8)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	0.0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	0.0	(2.9)
Comprehensive income (loss) attributable to White Mountains’s common shareholders	230.3	511.1	787.6
White Mountains Insurance Group Ltd.
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Revenues (loss) (including realized and unrealized gains and losses)	50.7	81.7	(0.4)
Expenses	51.8	60.5	67.1
Pre-tax income (loss) from continuing operations	(1.1)	21.2	(67.5)
Income tax (expense) benefit	(0.8)	(1.2)	(0.9)
Net income (loss) from continuing operations	(1.9)	20.0	(68.4)
Equity in earnings (losses) from consolidated and unconsolidated subsidiaries, net of tax	232.3	489.2	(42.0)
Equity in earnings from discontinued operations, net of tax - NSM Group	0.0	0.0	16.4
Net income (loss) attributable to White Mountains’s common shareholders	230.4	509.2	792.8
Other comprehensive income (loss), net of tax	(0.1)	1.9	(2.9)
Other comprehensive income (loss) from discontinued operations, net of tax - NSM Group	0.0	0.0	(5.2)
Net gain (loss) from foreign currency translation from sale of discontinued operations, net of tax - NSM Group	0.0	0.0	2.9
Comprehensive income (loss) attributable to White Mountains’s common shareholders	230.3	511.1	787.6
NSM
CONDENSED FINANCIAL INFORMATION OF THE REGISTRANT
Net gain (loss) from sale of discontinued operations, net of tax - Sirius Group	0.0	0.0	886.8
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	$ 0.0	$ 886.8