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, 2014	Dec. 31, 2013
Offsetting Liabilities [Line Items]
Derivative Assets	$ 86.8	$ 106.0
Derivative Liabilities	(12.0)	(36.8)
Net amounts recognized in Other assets	74.8	69.2
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.1	2.4
Derivative Liabilities	(4.2)	(11.7)
Net amounts recognized in Other assets	(4.1)	(9.3)
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	59.2	67.8
Derivative Liabilities	0	(12.0)
Net amounts recognized in Other assets	59.2	55.8
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	25.2	30.7
Derivative Liabilities	(4.7)	(9.2)
Net amounts recognized in Other assets	20.5	21.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	1.2	1.0
Derivative Liabilities	(0.4)	(1.6)
Net amounts recognized in Other assets	0.8	(0.6)
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.7	2.3
Derivative Liabilities	(1.7)	0
Net amounts recognized in Other assets	(1.0)	2.3
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.4	1.8
Derivative Liabilities	(1.0)	(2.3)
Net amounts recognized in Other assets	$ (0.6)	$ (0.5)