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Fair Value Measurements (Details) - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | $ 519.3 |
Total short-term investments | 570.0 |
Corporate notes and obligations |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 269.2 |
U.S. Treasury securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 175.9 |
Certificates of deposit |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 70.6 |
U.S. agency obligations |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 37.1 |
Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 17.2 |
Money market funds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 355.5 |
Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 27.4 |
U.S. Treasury securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 33.4 |
Fair Value, Measurements, Recurring |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 570.0 |
Total cash equivalents and short-term investments | 986.3 |
Fair Value, Measurements, Recurring | Corporate notes and obligations |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 269.2 |
Fair Value, Measurements, Recurring | U.S. Treasury securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 175.9 |
Fair Value, Measurements, Recurring | Certificates of deposit |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 70.6 |
Fair Value, Measurements, Recurring | U.S. agency obligations |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 37.1 |
Fair Value, Measurements, Recurring | Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 17.2 |
Fair Value, Measurements, Recurring | Money market funds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 355.5 |
Fair Value, Measurements, Recurring | Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 27.4 |
Fair Value, Measurements, Recurring | Cash equivalents |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 416.3 |
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 | Level 1 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 0.0 |
Total cash equivalents and short-term investments | 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 |
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 |
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 |
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 |
Fair Value, Measurements, Recurring | Level 1 | Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 0.0 |
Fair Value, Measurements, Recurring | Level 1 | Money market funds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 355.5 | $ 367.1
Fair Value, Measurements, Recurring | Level 1 | Commercial paper |  |
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 | 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 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 570.0 |
Total cash equivalents and short-term investments | 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 | 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 | 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 | 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 | 37.1 |
Fair Value, Measurements, Recurring | Level 2 | Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 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 |
Fair Value, Measurements, Recurring | Level 2 | Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 27.4 |
Fair Value, Measurements, Recurring | Level 2 | Cash equivalents |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total cash equivalents | 60.8 |
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 3 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 0.0 |
Total cash equivalents and short-term investments | 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 |
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 |
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 |
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 |
Fair Value, Measurements, Recurring | Level 3 | Commercial paper |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Total short-term investments | 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 |
Fair Value, Measurements, Recurring | Level 3 | Commercial paper |  |
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 |
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 |

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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