Derivatives Additional Collateral (Details) (White Mountains Life Re, USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Credit risk on derivative instruments
Collateral cash and investments	$ 36.5	$ 81.3	$ 229.8
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	22.2	56.1	188.5
Short-term investments
Credit risk on derivative instruments
Derivative collateral right to reclaim	0	2.0	15.6
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 14.3	$ 23.2	$ 25.7