Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Credit risk on derivative instruments
Derivative Counterparty Credit Risk	$ (5.2)		$ (9.1)
White Mountains Life Re [Member]
Credit risk on derivative instruments
Derivative Assets (Liabilities), at Fair Value, Net	251.4	[1]	295.4	[1]
Derivative Collateral Right to Reclaim	(63.3)		(73.2)
Derivative Collateral Obligation to Return	85.6		83.0
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	$ 273.7		$ 305.2

[1] (1) Value of OTC derivative instruments as of September 30, 2012 and December 31, 2011 excludes adjustments for counterparty credit risk of $(5.2) and $(9.1) included in fair value under GAAP.The following table