Equity-Method Eligible Investments - MediaAlpha Investment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Subsidiary, Sale of Stock [Line Items]
Total assets	$ 4,831.4				$ 3,983.2				$ 4,831.4	$ 3,983.2
Total liabilities	1,013.5				838.5				1,013.5	838.5
Total revenues	520.8	$ 459.5	$ 259.6	$ (59.2)	175.3	$ 155.5	$ 129.0	$ 433.6	1,180.7	893.4	$ 369.1
MediaAlpha
Subsidiary, Sale of Stock [Line Items]
Total assets	212.7				105.4				212.7	105.4
Total liabilities	$ 315.8				$ 144.9				315.8	144.9
Total revenues									584.8	408.0	296.9
Total expenses									575.4	390.2	278.8
Net income (loss) attributable to White Mountains’s common shareholders									$ 9.4	17.8	18.1
MediaAlpha | Total Assets
Subsidiary, Sale of Stock [Line Items]
Out of period adjustments										(1.5)
MediaAlpha | Total Liabilities
Subsidiary, Sale of Stock [Line Items]
Out of period adjustments										0.3
MediaAlpha | Total Revenues
Subsidiary, Sale of Stock [Line Items]
Out of period adjustments										0.1	(0.2)
MediaAlpha | Total Operating Expenses
Subsidiary, Sale of Stock [Line Items]
Out of period adjustments										1.2	(9.4)
MediaAlpha | Total Net Income (Loss)
Subsidiary, Sale of Stock [Line Items]
Out of period adjustments										$ (1.1)	$ 9.2