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Cash, Cash Equivalents and Short-Term Investments - Schedule of Cash, Cash Equivalents and Short-Term Investments (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Cash and Cash Equivalents [Line Items]    
Cash and cash equivalents $ 359.2 $ 519.3
Total cash equivalents 359.2 519.3
Short-term investments, amortized cost 555.5 570.5
Short-term investments, unrealized gain 0.6 0.1
Short-term investments, unrealized loss (0.1) (0.6)
Short-term investments, estimated fair value 556.0 570.0
Total cash, cash equivalents, and short term investments, before unrealized gains (losses) on investments 914.7 1,089.8
Total cash, cash equivalents, and short term investments 915.2 1,089.3
Corporate notes and obligations    
Cash and Cash Equivalents [Line Items]    
Short-term investments, amortized cost 244.7 269.6
Short-term investments, unrealized gain 0.6 0.1
Short-term investments, unrealized loss 0.0 (0.5)
Short-term investments, estimated fair value 245.3 269.2
U.S. Treasury securities    
Cash and Cash Equivalents [Line Items]    
Short-term investments, amortized cost 193.0 176.0
Short-term investments, unrealized gain 0.0 0.0
Short-term investments, unrealized loss (0.1) (0.1)
Short-term investments, estimated fair value 192.9 175.9
Certificates of deposit    
Cash and Cash Equivalents [Line Items]    
Short-term investments, amortized cost 52.2 70.6
Short-term investments, unrealized gain 0.0 0.0
Short-term investments, unrealized loss 0.0 0.0
Short-term investments, estimated fair value 52.2 70.6
U.S. agency obligations    
Cash and Cash Equivalents [Line Items]    
Short-term investments, amortized cost 38.5 37.1
Short-term investments, unrealized gain 0.0 0.0
Short-term investments, unrealized loss 0.0 0.0
Short-term investments, estimated fair value 38.5 37.1
Commercial paper    
Cash and Cash Equivalents [Line Items]    
Short-term investments, amortized cost 27.1 17.2
Short-term investments, unrealized gain 0.0 0.0
Short-term investments, unrealized loss 0.0 0.0
Short-term investments, estimated fair value 27.1 17.2
Cash    
Cash and Cash Equivalents [Line Items]    
Cash and cash equivalents 109.8 103.0
Total cash equivalents 109.8 103.0
Money market funds    
Cash and Cash Equivalents [Line Items]    
Cash and cash equivalents 226.9 355.5
Total cash equivalents 226.9 355.5
U.S. Treasury securities    
Cash and Cash Equivalents [Line Items]    
Cash and cash equivalents   33.4
Total cash equivalents   33.4
Commercial paper    
Cash and Cash Equivalents [Line Items]    
Cash and cash equivalents 19.5 27.4
Total cash equivalents 19.5 $ 27.4
Corporate notes and obligations    
Cash and Cash Equivalents [Line Items]    
Cash and cash equivalents 3.0  
Total cash equivalents $ 3.0