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Cash, Cash Equivalents and Short-Term Investments - Short-term Investments In Unrealized Loss Positions (Details)
$ in Millions
Dec. 31, 2018
USD ($)
Fair value  
Less than 12 months $ 337.3
12 months or greater 0.0
Total 337.3
Unrealized losses  
Less than 12 months (0.6)
12 months or greater 0.0
Total (0.6)
Corporate notes and obligations  
Fair value  
Less than 12 months 209.5
12 months or greater 0.0
Total 209.5
Unrealized losses  
Less than 12 months (0.5)
12 months or greater 0.0
Total (0.5)
U.S. Treasury securities  
Fair value  
Less than 12 months 127.8
12 months or greater 0.0
Total 127.8
Unrealized losses  
Less than 12 months (0.1)
12 months or greater 0.0
Total $ (0.1)