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Investments - Loss from Closed Currency Forward Contracts Recorded in Accumulated Other Comprehensive Income (Loss) - Narrative (Details) - USD ($)
$ in Millions
Jun. 30, 2017
Dec. 31, 2016
Derivative Instruments, Gain (Loss) Recognized in Other Comprehensive Income (Loss), Effective Portion, Net [Abstract]    
Deferred loss related to closed forward contracts $ (26.4) $ (26.4)