Segment Information (Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2012
Segment reporting information
Total investments	$ 2,947.2	$ 2,542.9
Assets	3,983.2	3,362.6
Liabilities	838.5	644.4
White Mountains’ common shareholders’ equity	3,261.5	2,843.1
Non-controlling interests	(116.8)	(124.9)
HG Global-BAM
Segment reporting information
Total investments	845.6	768.3
Assets	924.0	816.2
Liabilities	246.8	212.5
White Mountains’ common shareholders’ equity	809.5	759.8
Non-controlling interests	(132.3)	(156.1)
NSM Holding Co LLC
Segment reporting information
Total investments	0.0	0.0
Assets	825.2	623.6
Liabilities	401.1	314.8
White Mountains’ common shareholders’ equity	409.3	294.9
Non-controlling interests	14.8	13.9
Kudu
Segment reporting information
Total investments	266.6	0.0
Assets	290.5	0.0
Liabilities	57.0	0.0
White Mountains’ common shareholders’ equity	231.3	0.0
Non-controlling interests	2.2	0.0
MediaAlpha
Segment reporting information
Total investments	0.0	0.0
Assets	0.0	88.4
Liabilities	0.0	46.9
White Mountains’ common shareholders’ equity	0.0	25.3
Non-controlling interests	0.0	16.2
Other Operations
Segment reporting information
Total investments	1,835.0	1,774.6
Assets	1,940.5	1,831.1
Liabilities	133.6	70.2
White Mountains’ common shareholders’ equity	1,808.4	1,759.8
Non-controlling interests	(1.5)	1.1
Discontinued Operations, Held-for-sale
Segment reporting information
Total investments	0.0	0.0
Assets	3.0	3.3
Liabilities	0.0	0.0
White Mountains’ common shareholders’ equity	3.0	3.3
Non-controlling interests	0.0	0.0
BAM
Segment reporting information
Surplus notes	457.6	481.3
Accrued interest on surplus notes	162.7	143.7
HG Global
Segment reporting information
Non-controlling interests	14.4	14.5
Surplus notes			$ 503.0
Preferred Dividends Payable | HG Global
Segment reporting information
Liabilities	$ 330.3	$ 278.5