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Fair Value Measurements (Details) - USD ($)
$ in Millions
Mar. 31, 2019
Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents $ 359.2 $ 519.3
Total short-term investments 556.0 570.0
Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 245.3 269.2
U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 192.9 175.9
Certificates of deposit    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 52.2 70.6
U.S. agency obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 38.5 37.1
Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 27.1 17.2
Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 226.9 355.5
U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents   33.4
Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 19.5 27.4
Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 3.0  
Fair Value, Measurements, Recurring    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 556.0 570.0
Total cash equivalents and short-term investments 805.4 986.3
Fair Value, Measurements, Recurring | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 245.3 269.2
Fair Value, Measurements, Recurring | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 192.9 175.9
Fair Value, Measurements, Recurring | Certificates of deposit    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 52.2 70.6
Fair Value, Measurements, Recurring | U.S. agency obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 38.5 37.1
Fair Value, Measurements, Recurring | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 27.1 17.2
Fair Value, Measurements, Recurring | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 226.9 355.5
Fair Value, Measurements, Recurring | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents   33.4
Fair Value, Measurements, Recurring | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 19.5 27.4
Fair Value, Measurements, Recurring | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 3.0  
Fair Value, Measurements, Recurring | Cash equivalents    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 249.4 416.3
Fair Value, Measurements, Recurring | Level 1    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Total cash equivalents and short-term investments 226.9 355.5
Fair Value, Measurements, Recurring | Level 1 | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 1 | Certificates of deposit    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 1 | U.S. agency obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 1 | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 226.9 355.5
Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents   0.0
Fair Value, Measurements, Recurring | Level 1 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 0.0 0.0
Fair Value, Measurements, Recurring | Level 1 | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 0.0  
Fair Value, Measurements, Recurring | Level 1 | Cash equivalents    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 226.9 355.5
Fair Value, Measurements, Recurring | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 556.0 570.0
Total cash equivalents and short-term investments 578.5 630.8
Fair Value, Measurements, Recurring | Level 2 | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 245.3 269.2
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 192.9 175.9
Fair Value, Measurements, Recurring | Level 2 | Certificates of deposit    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 52.2 70.6
Fair Value, Measurements, Recurring | Level 2 | U.S. agency obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 38.5 37.1
Fair Value, Measurements, Recurring | Level 2 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 27.1 17.2
Fair Value, Measurements, Recurring | Level 2 | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 0.0 0.0
Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents   33.4
Fair Value, Measurements, Recurring | Level 2 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 19.5 27.4
Fair Value, Measurements, Recurring | Level 2 | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 3.0  
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 22.5 60.8
Fair Value, Measurements, Recurring | Level 3    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Total cash equivalents and short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | Certificates of deposit    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | U.S. agency obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total short-term investments 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | Money market funds    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents   0.0
Fair Value, Measurements, Recurring | Level 3 | Commercial paper    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 0.0 0.0
Fair Value, Measurements, Recurring | Level 3 | Corporate notes and obligations    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents 0.0  
Fair Value, Measurements, Recurring | Level 3 | Cash equivalents    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Total cash equivalents $ 0.0 $ 0.0