Derivatives (Collateral Provided to WM Life Re from Counterparties) (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Variable Annuity Reinsurance [Line Items]
Collateral provided	$ 119.3	$ 83.0
Derivative Collateral Right to Reclaim	30.6	73.2
Fixed maturity investments
Variable Annuity Reinsurance [Line Items]
Derivative Collateral Right to Reclaim	138.7	31.2
Short-term investments
Variable Annuity Reinsurance [Line Items]
Derivative Collateral Right to Reclaim	$ 5.1	$ 0.6