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	Sep. 30, 2014	Dec. 31, 2013
Offsetting Liabilities [Line Items]
Derivative Assets	$ 75.1	$ 106.0
Derivative Liabilities	(9.8)	(36.8)
Net amounts recognized in Other assets	65.3	69.2
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.2	2.4
Derivative Liabilities	(3.1)	(11.7)
Net amounts recognized in Other assets	(2.9)	(9.3)
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	48.0	67.8
Derivative Liabilities	0	(12.0)
Net amounts recognized in Other assets	48.0	55.8
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	20.5	30.7
Derivative Liabilities	(4.7)	(9.2)
Net amounts recognized in Other assets	15.8	21.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.8	1.0
Derivative Liabilities	0	(1.6)
Net amounts recognized in Other assets	0.8	(0.6)
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	5.0	2.3
Derivative Liabilities	(0.7)	0
Net amounts recognized in Other assets	4.3	2.3
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.6	1.8
Derivative Liabilities	(1.3)	(2.3)
Net amounts recognized in Other assets	$ (0.7)	$ (0.5)