Segment Information (Assets) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2012
Segment reporting information
Total investments	$ 2,542.9	$ 3,380.7
Assets	3,362.6	3,659.2
Liabilities	644.4	298.4
White Mountains’ common shareholders’ equity	2,843.1	3,492.5
Non-controlling interests	(124.9)	(131.7)
NSM Holding Co LLC
Segment reporting information
Total investments	1.7	0.0
Assets	627.0	0.0
Liabilities	314.8	0.0
White Mountains’ common shareholders’ equity	298.3	0.0
Non-controlling interests	13.9	0.0
MediaAlpha
Segment reporting information
Total investments	0.0	0.0
Assets	88.4	96.5
Liabilities	46.9	59.8
White Mountains’ common shareholders’ equity	25.3	23.6
Non-controlling interests	16.2	13.1
Discontinued Operations, Held-for-sale
Segment reporting information
Total investments	0.0	0.0
Assets	3.3	3.3
Liabilities	0.0	0.0
White Mountains’ common shareholders’ equity	3.3	3.3
Non-controlling interests	0.0	0.0
HG Global-BAM
Segment reporting information
Total investments	768.3	693.4
Assets	816.2	747.4
Liabilities	212.5	167.0
White Mountains’ common shareholders’ equity	759.8	727.7
Non-controlling interests	(156.1)	(147.3)
Other Operations
Segment reporting information
Total investments	1,772.9	2,687.3
Assets	1,827.7	2,812.0
Liabilities	70.2	71.6
White Mountains’ common shareholders’ equity	1,756.4	2,737.9
Non-controlling interests	1.1	2.5
BAM
Segment reporting information
Surplus notes	481.3	499.0
Accrued interest on surplus notes	143.7	126.0
HG Global
Segment reporting information
Non-controlling interests	14.5	15.9
Surplus notes			$ 503.0
Preferred Dividends Payable | HG Global
Segment reporting information
Liabilities	$ 278.5	$ 227.9