Derivatives (Realized and Unrealized Gains (Losses)) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Variable Annuity Reinsurance [Line Items]
Change in fair value of variable annuity death benefit liabilities, included in general and administrative expenses	$ 10.2		$ 14.2		$ (1.8)
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	(402.0)		(339.0)		92.9
Derivative, Fair Value, Net	69.2		98.3
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	(108.7)		(149.5)		8.9
Derivative, Fair Value, Net	(9.7)		27.1
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	(96.7)		(102.3)		29.5
Derivative, Fair Value, Net	58.0		52.8
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	(196.6)		(87.2)		54.5
Derivative, Fair Value, Net	20.9		18.4
Level 3
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	136.5	[1]	84.0	[1]	(14.5)	[1]
Derivative, Fair Value, Net	63.4	[1]	140.5	[1]	247.1	[1]	275.3	[1]
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	$ 6.5				$ 7.2

[1] Consists of over-the-counter instruments.