Derivatives Derivatives (Gross Asset (Gross liability) Amounts Offset Under Master Netting Agreements) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Derivative [Line Items]
Derivative Asset	$ 64.4	$ 106.0
Derivative Liability	(8.0)	(36.8)
Net amounts recognized in Other Assets	56.4	69.2
Interest Rate Contract | Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	1.0	2.4
Derivative Liability	(5.4)	(11.7)
Net amounts recognized in Other Assets	(4.4)	(9.3)
Interest Rate Contract | Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	2.8	1.0
Derivative Liability	(0.1)	(1.6)
Net amounts recognized in Other Assets	2.7	(0.6)
Foreign Exchange Contract | Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	45.5	67.8
Derivative Liability	0	(12.0)
Net amounts recognized in Other Assets	45.5	55.8
Foreign Exchange Contract | Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	0	2.3
Derivative Liability	(1.4)	0
Net amounts recognized in Other Assets	(1.4)	2.3
Equity Contract | Over the Counter [Member]
Derivative [Line Items]
Derivative Asset	11.7	30.7
Derivative Liability	(0.2)	(9.2)
Net amounts recognized in Other Assets	11.5	21.5
Equity Contract | Exchange Traded [Member]
Derivative [Line Items]
Derivative Asset	3.4	1.8
Derivative Liability	(0.9)	(2.3)
Net amounts recognized in Other Assets	$ 2.5	$ (0.5)