Derivatives (Collateral Provided to WM Life Re from Counterparties) (Details) (White Mountains Life Re [Member], USD $) In Millions, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Variable Annuity Reinsurance [Line Items]
Derivative Collateral Obligation to Return	$ 85.6	$ 83.0
Derivative Collateral Right to Reclaim	63.3	73.2
Short-term investments
Variable Annuity Reinsurance [Line Items]
Derivative Collateral Obligation to Return	63.3	73.2
Derivative Collateral Right to Reclaim	14.6	0.6
Fixed maturity investments.
Variable Annuity Reinsurance [Line Items]
Derivative Collateral Obligation to Return	0	0
Derivative Collateral Right to Reclaim	$ 121.2	$ 31.2