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Cash, Cash Equivalents and Short-Term Investments - Short-term Investments In Unrealized Loss Positions (Details) $ in Millions | Dec. 31, 2018 USD ($)
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Fair value |
Less than 12 months | $ 337.3
12 months or greater | 0.0
Total | 337.3
Unrealized losses |
Less than 12 months | (0.6)
12 months or greater | 0.0
Total | (0.6)
Corporate notes and obligations |
Fair value |
Less than 12 months | 209.5
12 months or greater | 0.0
Total | 209.5
Unrealized losses |
Less than 12 months | (0.5)
12 months or greater | 0.0
Total | (0.5)
U.S. Treasury securities |
Fair value |
Less than 12 months | 127.8
12 months or greater | 0.0
Total | 127.8
Unrealized losses |
Less than 12 months | (0.1)
12 months or greater | 0.0
Total | $ (0.1)

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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), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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