Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Credit risk on derivative instruments
Collateral cash and investments	$ 42.3	$ 81.3	$ 120.6
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	27.8	56.1	61.2
Short-term investments
Credit risk on derivative instruments
Derivative collateral right to reclaim	0	2.0	35.1
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 14.5	$ 23.2	$ 24.3