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Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | $ 37,638 |  | $ 36,549 |
Money Market Funds |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [1] | 14,334 |  | 10,156 |
Restricted cash (money market funds) |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 5,333 | [2] | 19,593 | [3]
U.S. Government Agency Securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | 16,471 |  | 6,800 |
Agency Securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | 1,500 |  |  |
Fair Value, Inputs, Level 1 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 36,138 |  | 36,549 |
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] | 14,334 |  | 10,156 |
Fair Value, Inputs, Level 1 | Restricted cash (money market funds) |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 5,333 | [2] | 19,593 | [3]
Fair Value, Inputs, Level 1 | U.S. Government Agency Securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | 16,471 |  | $ 6,800 |
Fair Value, Inputs, Level 2 |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value |  | 1,500 |  |  |
Fair Value, Inputs, Level 2 | Agency Securities |  |  |  |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |
Assets, fair value | [4] | $ 1,500 |  |  |
[1] | Included in cash and cash equivalents on accompanying consolidated balance sheets.
[2] | $5.2 million of restricted cash in the Indenture serves as full collateral for the redeemable convertible notes and $0.1 million of restricted cash serves as collateral for the Company’s corporate credit card and deposit for its facility lease.
[3] | $19.4 million of restricted cash in the Indenture serves as full collateral for the redeemable convertible notes and $0.1 million of restricted cash serves as collateral for the Company’s corporate credit card and deposit for its facility lease.
[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://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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