Equity-Method Eligible Investments - Equity Method Investments Financial Data (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Schedule of Equity Method Investments [Line Items]
Total assets	$ 4,383.4		$ 4,383.4		$ 3,983.2
Total liabilities	1,074.2		1,074.2		838.5
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
MediaAlpha, Kudu and Tuckerman
Schedule of Equity Method Investments [Line Items]
Total assets	135.7		135.7		105.4
Total liabilities	460.8		460.8		$ 144.9
Total revenues	151.5	110.4	394.6	281.9
Total expenses	(144.1)	(102.6)	(368.2)	(274.5)
Net income attributable to White Mountains’s common shareholders	$ 7.4	$ 7.8	$ 26.4	$ 7.4