Equity-Method Eligible Investments - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			7 Months Ended	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Feb. 26, 2019	Feb. 25, 2019
Schedule of Equity Method Investments [Line Items]
Total revenues	$ 459.5	$ 155.5			$ 659.9	$ 718.1
Net income attributable to White Mountains’s common shareholders	$ 232.9	48.7			$ 219.5	353.6
Marketing Technology (MediaAlpha)
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	48.30%				48.30%		48.30%	48.30%	61.00%
Fully diluted ownership interest (as a percent)								42.00%	58.90%
Assets, fair value			$ 114.7
Unrealized investment gain	$ 250.0	$ 35.3	$ 114.7		$ 295.0	$ 150.0
Total revenues				$ 233.1
Total expenses				220.6
Net income attributable to White Mountains’s common shareholders				$ 12.5