Derivatives (Value, Collateral (held) provided by WM Life Re and Net Exposure to Credit Losses on OTC Derivative Instruments) (Details) - White Mountains Life Re $ in Millions	Dec. 31, 2015 USD ($)
Offsetting Liabilities [Line Items]
Derivative Assets	$ 23.2
Derivative Liabilities	(3.1)
Net amounts recognized in Other assets	20.1
Over The Counter | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	2.4
Derivative Liabilities	(2.1)
Net amounts recognized in Other assets	0.3
Over The Counter | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	15.0
Derivative Liabilities	0.0
Net amounts recognized in Other assets	15.0
Over The Counter | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	4.4
Derivative Liabilities	(0.6)
Net amounts recognized in Other assets	3.8
Exchange Traded | Fixed income/interest rate
Offsetting Liabilities [Line Items]
Derivative Assets	0.1
Derivative Liabilities	(0.1)
Net amounts recognized in Other assets	0.0
Exchange Traded | Foreign exchange
Offsetting Liabilities [Line Items]
Derivative Assets	0.1
Derivative Liabilities	(0.3)
Net amounts recognized in Other assets	(0.2)
Exchange Traded | Equity
Offsetting Liabilities [Line Items]
Derivative Assets	1.2
Derivative Liabilities	0.0
Net amounts recognized in Other assets	$ 1.2