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, 2015	Dec. 31, 2014
Offsetting Liabilities [Line Items]
Derivative Assets	$ 52.3	$ 64.4
Derivative Liabilities	(5.3)	(8.0)
Net amounts recognized in Other assets	47.0	56.4
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	3.4	1.0
Derivative Liabilities	(0.9)	(5.4)
Net amounts recognized in Other assets	2.5	(4.4)
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	38.9	45.5
Derivative Liabilities	0	0
Net amounts recognized in Other assets	38.9	45.5
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	7.3	11.7
Derivative Liabilities	(0.4)	(0.2)
Net amounts recognized in Other assets	6.9	11.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	1.1	2.8
Derivative Liabilities	(0.3)	(0.1)
Net amounts recognized in Other assets	0.8	2.7
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0	0
Derivative Liabilities	(3.6)	(1.4)
Net amounts recognized in Other assets	(3.6)	(1.4)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	1.6	3.4
Derivative Liabilities	(0.1)	(0.9)
Net amounts recognized in Other assets	$ 1.5	$ 2.5