Derivatives (Uncollateralized Amounts Due Under WM Life Re's OTC Derivative Contracts) (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012 Rating		Dec. 31, 2011
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	$ 273.7		$ 305.2
S&P Credit Ratings	22
Citigroup [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	63.4	[1]
Royal Bank of Scotland [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	58.5
Bank of America [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	50.3
J P Morgan Chase [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	40.8	[1]
Nomura [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	35.9	[1]
Barclays [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	20.7
Goldman Sachs [Member]
Credit risk on derivative instruments
Net Exposure On Fair Value Of Over The Counter Derivative Instruments	$ 4.1	[1]

[1] (2) Collateral provided (held) calculated under master netting agreement.