Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2013
Credit risk on derivative instruments
Collateral cash and investments	$ 34.2	$ 81.3	$ 114.8
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	23.8	56.1	74.4
Short-term investments
Credit risk on derivative instruments
Derivative collateral right to reclaim	0	2.0	15.8
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 10.4	$ 23.2	$ 24.6