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, 2014	Dec. 31, 2013
Offsetting Liabilities [Line Items]
Derivative Assets	$ 99.7	$ 106.0
Derivative Liabilities	(10.1)	(36.8)
Net amounts recognized in Other assets	89.6	69.2
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	1.2	2.4
Derivative Liabilities	(6.4)	(11.7)
Net amounts recognized in Other assets	(5.2)	(9.3)
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	64.4	67.8
Derivative Liabilities	0	(12.0)
Net amounts recognized in Other assets	64.4	55.8
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	32.6	30.7
Derivative Liabilities	(2.5)	(9.2)
Net amounts recognized in Other assets	30.1	21.5
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.5	1.0
Derivative Liabilities	(0.6)	(1.6)
Net amounts recognized in Other assets	(0.1)	(0.6)
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.1	2.3
Derivative Liabilities	(0.6)	0
Net amounts recognized in Other assets	(0.5)	2.3
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.9	1.8
Derivative Liabilities	0	(2.3)
Net amounts recognized in Other assets	$ 0.9	$ (0.5)