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Fair Value Measurements and Available for Sale Investments - Assets Measured at Fair Value on Recurring Basis (Details) - Recurring Basis - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
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Money Market Funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | $ 35,756 | $ 17,295
U S Treasury and Agency Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | 199,500 |
Commercial Paper |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | 30,985 |
Corporate Debt Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | 99,892 |
U.S. Treasury Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis |  | 123,439
Quoted Market Prices for Identical Assets (Level 1) | Money Market Funds |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | 35,756 | 17,295
Quoted Market Prices for Identical Assets (Level 1) | U S Treasury and Agency Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | 199,500 |
Quoted Market Prices for Identical Assets (Level 1) | U.S. Treasury Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis |  | $ 123,439
Significant Other Observable Inputs (Level 2) | Commercial Paper |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | 30,985 |
Significant Other Observable Inputs (Level 2) | Corporate Debt Securities |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Assets Measured at fair value on recurring basis | $ 99,892 |

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258 + Details
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