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