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	Mar. 31, 2016	Dec. 31, 2015
Offsetting Liabilities [Line Items]
Derivative Assets	$ 16.7	$ 23.2
Derivative Liabilities	(3.6)	(3.1)
Net amounts recognized in Other assets	13.1	20.1
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	2.2	2.4
Derivative Liabilities	(2.5)	(2.1)
Net amounts recognized in Other assets	(0.3)	0.3
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	12.7	15.0
Derivative Liabilities	0.0	0.0
Net amounts recognized in Other assets	12.7	15.0
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	1.5	4.4
Derivative Liabilities	(0.9)	(0.6)
Net amounts recognized in Other assets	0.6	3.8
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.0	0.1
Derivative Liabilities	0.0	(0.1)
Net amounts recognized in Other assets	0.0	0.0
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.0	0.1
Derivative Liabilities	(0.1)	(0.3)
Net amounts recognized in Other assets	(0.1)	(0.2)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	0.3	1.2
Derivative Liabilities	(0.1)	0.0
Net amounts recognized in Other assets	$ 0.2	$ 1.2