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Investments - Available-for-sale (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Marketable investments classified as available-for-sale:    
Amortized Cost $ 481.0  
Gross Unrealized Losses (0.5)  
Fair Value 480.5 $ 0.0
U.S. government and agency securities    
Marketable investments classified as available-for-sale:    
Amortized Cost 481.0  
Gross Unrealized Losses (0.5)  
Fair Value $ 480.5