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Fair Value Measurements and Marketable Securities - Summary of Financial Assets and Liabilities Measured at Fair Value Recurring Basis (Detail) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
Cash equivalents: |  |  |
Total cash equivalents | $ 48,079 | $ 83,121 |
Marketable securities: |  |  |
Total marketable securities | 22,841 |  |
Total cash equivalents and marketable securities | 70,920 |  |
Derivative liabilities: |  |  |
Derivative Liabilities | 223 | 223 | $ 2,708
Anti- dilution Rights [Member] |  |  |
Derivative liabilities: |  |  |
Derivative liabilities | 223 | 223 | 1,806
Contingent prepayment option [Member] |  |  |
Derivative liabilities: |  |  |
Derivative liabilities |  |  | 902
Money Market Funds [Member] |  |  |
Cash equivalents: |  |  |
Total cash equivalents | 44,083 | 83,121 |
U.S. Commercial Paper [Member] |  |  |
Cash equivalents: |  |  |
Total cash equivalents | 3,996 |  |
Marketable securities: |  |  |
Total marketable securities | 5,946 |  |
U.S. Government Securities [Member] |  |  |
Marketable securities: |  |  |
Total marketable securities | 8,919 |  |
U.S. Corporate Bonds [Member] |  |  |
Marketable securities: |  |  |
Total marketable securities | 7,976 |  |
Level 2 [Member] |  |  |
Cash equivalents: |  |  |
Total cash equivalents | 48,079 | 83,121 |
Marketable securities: |  |  |
Total marketable securities | 22,841 |  |
Total cash equivalents and marketable securities | 70,920 |  |
Level 2 [Member] | Money Market Funds [Member] |  |  |
Cash equivalents: |  |  |
Total cash equivalents | 44,083 | 83,121 |
Level 2 [Member] | U.S. Commercial Paper [Member] |  |  |
Cash equivalents: |  |  |
Total cash equivalents | 3,996 |  |
Marketable securities: |  |  |
Total marketable securities | 5,946 |  |
Level 2 [Member] | U.S. Government Securities [Member] |  |  |
Marketable securities: |  |  |
Total marketable securities | 8,919 |  |
Level 2 [Member] | U.S. Corporate Bonds [Member] |  |  |
Marketable securities: |  |  |
Total marketable securities | 7,976 |  |
Level 3 [Member] |  |  |
Derivative liabilities: |  |  |
Derivative Liabilities | 223 | 223 | 2,708
Level 3 [Member] | Anti- dilution Rights [Member] |  |  |
Derivative liabilities: |  |  |
Derivative liabilities | $ 223 | $ 223 | 1,806
Level 3 [Member] | Contingent prepayment option [Member] |  |  |
Derivative liabilities: |  |  |
Derivative liabilities |  |  | $ 902

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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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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References No definition available. + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=82849420&loc=SL20226008-175313 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13495-108611 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=77997519&loc=d3e13433-108611 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 + Details
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