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Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | $ 119,053 |  | $ 37,638 |
Money Market Funds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [1] | 29,090 |  | 14,334 |
Restricted cash |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 50 | [2] | 5,333 | [3]
Federal Agency Securities | Cash and Cash Equivalents |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [1] | 999 |  |  |
Federal Agency Securities | Short-Term Investments |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | 14,775 |  | 1,500 |
U.S. Treasury Securities | Short-Term Investments |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | 74,139 |  | 16,471 |
Fair Value, Inputs, Level 1 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 103,279 |  | 36,138 |
Fair Value, Inputs, Level 1 | Money Market Funds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [1] | 29,090 |  | 14,334 |
Fair Value, Inputs, Level 1 | Restricted cash |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 50 | [2] | 5,333 | [3]
Fair Value, Inputs, Level 1 | U.S. Treasury Securities | Short-Term Investments |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | 74,139 |  | 16,471 |
Fair Value, Inputs, Level 2 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 15,774 |  | 1,500 |
Fair Value, Inputs, Level 2 | Federal Agency Securities | Cash and Cash Equivalents |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [1] | 999 |  |  |
Fair Value, Inputs, Level 2 | Federal Agency Securities | Short-Term Investments |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | $ 14,775 |  | $ 1,500 |
[1] | Included in cash and cash equivalents on accompanying consolidated balance sheets.
[2] | $0.05 million of restricted cash serves as collateral for the Company’s corporate credit card.
[3] | $5.3 million of restricted cash in the Indenture (see Note 9) serves as full collateral for the redeemable convertible notes.
[4] | Included in short-term investments on accompanying consolidated balance sheets and are classified as available-for-sale securities.

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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=99377789&loc=d3e19207-110258 + Details
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