Derivatives (Realized and Unrealized Gains (Losses)) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ (72.4)	$ (402.0)	$ (339.0)
Derivative, Fair Value, Net	56.4	69.2	98.3	290.4
Fixed income/interest rate
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(33.7)	(108.7)	(149.5)
Derivative, Fair Value, Net	(1.7)	(9.7)
Foreign exchange
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(1.3)	(96.7)	(102.3)
Derivative, Fair Value, Net	44.1	58.0
Equity
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	(37.4)	(196.6)	(87.2)
Derivative, Fair Value, Net	14.0	20.9
Level 3 | Projected Surrender Assumption [Member]
Realized and unrealized derivative gains (losses) recognized in other revenues and carrying values of instruments
Change in fair value of derivatives, included in other revenue	$ (0.2)	$ 7.2