Investments in Unconsolidated Entities - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 31, 2020	Feb. 26, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		53.80%			53.80%		50.00%	50.00%
Total revenues		$ 259,600,000	$ 129,000,000.0		$ 200,400,000	$ 562,600,000
Benefits, Losses and Expenses		126,900,000	113,200,000		233,700,000	265,100,000
Total assets		4,000,300,000			4,000,300,000		$ 3,983,200,000
Total liabilities		937,000,000.0			937,000,000.0		838,500,000
Net income (loss) attributable to White Mountains’s common shareholders		115,400,000	20,500,000		(13,400,000)	304,900,000
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)									48.30%	61.00%
Fully diluted ownership interest (as a percent)									42.00%	58.90%
Assets, fair value	$ 114,700,000			$ 114,700,000					$ 114,700,000
Unrealized investment gain		15,000,000.0		$ 114,700,000	45,000,000.0	114,700,000	180,000,000.0
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Total revenues		0	0		0	48,800,000
Benefits, Losses and Expenses		0	0		0	54,900,000
MediaAlpha, Kudu and Tuckerman
Schedule of Equity Method Investments [Line Items]
Total revenues		118,700,000	93,700,000		307,200,000	178,400,000
Expenses		131,900,000	83,600,000		273,800,000	171,100,000
Total assets		420,500,000			420,500,000		457,000,000.0
Total liabilities		200,200,000			200,200,000		$ 199,000,000.0
Net income (loss) attributable to White Mountains’s common shareholders		$ (21,800,000)	$ 10,100,000		$ 21,800,000	$ 7,300,000
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)		48.30%			48.30%		48.30%
Total revenues	122,700,000
Benefits, Losses and Expenses	115,800,000
Net income (loss) attributable to White Mountains’s common shareholders	$ 6,900,000