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Balance Sheet Details - Short-term Marketable Securities (Details) - USD ($)
$ in Millions
Mar. 31, 2020
Dec. 31, 2019
Debt securities, available for sale:    
Amortized Cost $ 923.2 $ 1,086.7
Gross Unrealized Gains 3.6 0.5
Gross Unrealized Losses (0.9) (0.1)
Estimated Market Value 925.9 1,087.1
U.S. government agencies    
Debt securities, available for sale:    
Amortized Cost 620.4 675.6
Gross Unrealized Gains 3.5 0.4
Gross Unrealized Losses 0.0 0.0
Estimated Market Value 623.9 676.0
Commercial paper    
Debt securities, available for sale:    
Amortized Cost 189.1 248.1
Gross Unrealized Gains 0.1 0.1
Gross Unrealized Losses (0.1) 0.0
Estimated Market Value 189.1 248.2
Corporate debt    
Debt securities, available for sale:    
Amortized Cost 113.7 163.0
Gross Unrealized Gains 0.0 0.0
Gross Unrealized Losses (0.8) (0.1)
Estimated Market Value $ 112.9 $ 162.9