Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Credit risk on derivative instruments
Collateral cash and investments	$ 8.4	$ 5.8	$ 33.5
Cash
Credit risk on derivative instruments
Derivative collateral right to reclaim	8.4	5.8	29.0
Fixed maturity investments.
Credit risk on derivative instruments
Derivative collateral right to reclaim	$ 0.0	$ 0.0	$ 4.5