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Financial Instruments and Fair Value , Fair Value Measurements (Details) - USD ($) $ in Millions | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | $ 947.2 | $ 987.4
Corporate debt securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 612.4 | 577.0
Commercial paper [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 150.6 | 148.8
Certificates of deposit and term deposits [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 87.1 | 66.4
U.S. government securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 97.1 | 195.2
Recurring Basis [Member] | Fair Value [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Assets, Fair Value Disclosure | 1,125.9 | 1,019.4
Recurring Basis [Member] | Fair Value [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Assets, Fair Value Disclosure | 260.8 | 210.2
Recurring Basis [Member] | Fair Value [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Assets, Fair Value Disclosure | 865.1 | 809.2
Recurring Basis [Member] | Fair Value [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Assets, Fair Value Disclosure | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Corporate debt securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 613.4 | 577.0
Recurring Basis [Member] | Fair Value [Member] | Corporate debt securities [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Corporate debt securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 613.4 | 577.0
Recurring Basis [Member] | Fair Value [Member] | Corporate debt securities [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Commercial paper [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 164.6 | 165.8
Recurring Basis [Member] | Fair Value [Member] | Commercial paper [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Commercial paper [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 164.6 | 165.8
Recurring Basis [Member] | Fair Value [Member] | Commercial paper [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Certificates of deposit and term deposits [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 87.1 | 66.4
Recurring Basis [Member] | Fair Value [Member] | Certificates of deposit and term deposits [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Certificates of deposit and term deposits [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 87.1 | 66.4
Recurring Basis [Member] | Fair Value [Member] | Certificates of deposit and term deposits [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | U.S. government securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 97.1 | 195.2
Recurring Basis [Member] | Fair Value [Member] | U.S. government securities [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 97.1 | 195.2
Recurring Basis [Member] | Fair Value [Member] | U.S. government securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | U.S. government securities [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Money market funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 163.7 | 15.0
Recurring Basis [Member] | Fair Value [Member] | Money market funds [Member] | Level 1 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 163.7 | 15.0
Recurring Basis [Member] | Fair Value [Member] | Money market funds [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Fair Value [Member] | Money market funds [Member] | Level 3 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 0.0 | 0.0
Recurring Basis [Member] | Reported as [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Total Assets, Fair Value Disclosure | 1,125.9 | 1,019.4
Recurring Basis [Member] | Reported as [Member] | Cash equivalents [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 178.7 | 32.0
Recurring Basis [Member] | Reported as [Member] | Short-term investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | 775.2 | 843.1
Recurring Basis [Member] | Reported as [Member] | Long-term investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Fair Value | $ 172.0 | $ 144.3

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