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	Mar. 31, 2013		Dec. 31, 2012
Offsetting Liabilities [Line Items]
Derivative Assets	$ 144.8	[1],[2]	$ 206.5	[1],[2]
Derivative Liabilities	(69.0)	[2]	(108.2)	[2]
Net amounts recognized in Other assets	75.8	[2]	98.3	[2]
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0	[1]	52.6	[1]
Derivative Liabilities	(15.6)		(26.9)
Net amounts recognized in Other assets	(15.6)		25.7
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	89.6	[1]	87.8	[1]
Derivative Liabilities	(15.5)		(34.4)
Net amounts recognized in Other assets	74.1		53.4
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	53.2	[1]	63.6	[1]
Derivative Liabilities	(27.2)		(22.9)
Net amounts recognized in Other assets	26.0		40.7
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	1.0	[1]	1.6	[1]
Derivative Liabilities	(1.6)		(0.2)
Net amounts recognized in Other assets	(0.6)		1.4
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.6	[1]	0.8	[1]
Derivative Liabilities	(0.6)		(1.4)
Net amounts recognized in Other assets	0		(0.6)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.4	[1]	0.1	[1]
Derivative Liabilities	(8.5)		(22.4)
Net amounts recognized in Other assets	$ (8.1)		$ (22.3)

[1] Amount equal to fair value of instrument as recognized in other assets.

[2] All derivative instruments held by WM Life Re are subject to master netting arrangements.