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	Jun. 30, 2013		Dec. 31, 2012
Offsetting Liabilities [Line Items]
Derivative Assets	$ 171.2	[1],[2]	$ 206.5	[1],[2]
Derivative Liabilities	(29.9)	[1]	(108.2)	[1]
Net amounts recognized in Other assets	141.3	[1]	98.3	[1]
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	12.8	[2]	52.6	[2]
Derivative Liabilities	(4.8)		(26.9)
Net amounts recognized in Other assets	8.0		25.7
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	87.3	[2]	87.8	[2]
Derivative Liabilities	(15.4)		(34.4)
Net amounts recognized in Other assets	71.9		53.4
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	64.9	[2]	63.6	[2]
Derivative Liabilities	0		(22.9)
Net amounts recognized in Other assets	64.9		40.7
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	4.3	[2]	1.6	[2]
Derivative Liabilities	(0.8)		(0.2)
Net amounts recognized in Other assets	3.5		1.4
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	1.2	[2]	0.8	[2]
Derivative Liabilities	(2.4)		(1.4)
Net amounts recognized in Other assets	(1.2)		(0.6)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.7	[2]	0.1	[2]
Derivative Liabilities	(6.5)		(22.4)
Net amounts recognized in Other assets	$ (5.8)		$ (22.3)

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

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