Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
White Mountains Life Re
Credit risk on derivative instruments
OTC derivative instruments	$ 123.5	[1]	$ 295.4	[1]
Collateral held	(30.6)		(73.2)
Collateral provided	119.3		83.0
Net exposure on fair value of OTC instruments	212.2		305.2
Derivative counterparty credit risk	$ (3.6)		$ (9.1)

[1] Value of OTC derivative instruments as of December 31, 2012 and 2011 excludes adjustments for counterparty credit risk of $(3.6) and $(9.1) included in fair value under GAAP.