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Marketable Investments - Available-for-sale (Details) - USD ($)
$ in Millions
Mar. 31, 2017
Dec. 31, 2016
Marketable investments classified as available-for-sale:    
Amortized Cost $ 311.8  
Gross Unrealized Gains 0.1  
Fair Value 311.9 $ 0.0
U.S. government and agency securities    
Marketable investments classified as available-for-sale:    
Amortized Cost 311.8  
Gross Unrealized Gains 0.1  
Fair Value $ 311.9