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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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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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