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	Jun. 30, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Derivative Assets	$ 44.2	$ 64.4
Derivative Liabilities	(8.4)	(8.0)
Net amounts recognized in Other assets	35.8	56.4
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	2.2	1.0
Derivative Liabilities	(2.1)	(5.4)
Net amounts recognized in Other assets	0.1	(4.4)
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	31.6	45.5
Derivative Liabilities	0.0	0.0
Net amounts recognized in Other assets	31.6	45.5
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	10.2	11.7
Derivative Liabilities	(1.2)	(0.2)
Net amounts recognized in Other assets	9.0	11.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.2	2.8
Derivative Liabilities	(0.6)	(0.1)
Net amounts recognized in Other assets	(0.4)	2.7
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.0	0.0
Derivative Liabilities	(2.3)	(1.4)
Net amounts recognized in Other assets	(2.3)	(1.4)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.0	3.4
Derivative Liabilities	(2.2)	(0.9)
Net amounts recognized in Other assets	$ (2.2)	$ 2.5