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Derivatives Additional Collateral (Details) - White Mountains Life Re - USD ($)
$ in Millions
Dec. 31, 2015
Sep. 30, 2015
Credit risk on derivative instruments    
Collateral cash and investments $ 5.8 $ 16.8
Cash    
Credit risk on derivative instruments    
Derivative collateral right to reclaim 5.8 16.8
Fixed maturity investments.    
Credit risk on derivative instruments    
Derivative collateral right to reclaim $ 0.0 $ 0.0