Investments and Fair Value of Financial Instruments - Summary of Fair Value of Financial Assets (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets at Fair Value
Total Assets at Fair Value	$ 17,281	$ 21,072
Money Market Funds [Member]
Assets at Fair Value
Cash equivalents	6,283	9,902
Commercial Paper [Member]
Assets at Fair Value
Cash equivalents	3,044	1,345
Short-term investments	5,110	6,776
Treasury Bills [Member]
Assets at Fair Value
Short-term investments	997
U.S. Government Securities [Member]
Assets at Fair Value
Short-term investments		1,198
Corporate Securities [Member]
Assets at Fair Value
Cash equivalents	599
Short-term investments	1,248	656
Treasury Bills [Member]
Assets at Fair Value
Short-term investments		1,195
Level 1 [Member]
Assets at Fair Value
Total Assets at Fair Value	7,280	12,295
Level 1 [Member] \| Money Market Funds [Member]
Assets at Fair Value
Cash equivalents	6,283	9,902
Level 1 [Member] \| Commercial Paper [Member]
Assets at Fair Value
Cash equivalents	0	0
Short-term investments	0	0
Level 1 [Member] \| Treasury Bills [Member]
Assets at Fair Value
Short-term investments	997
Level 1 [Member] \| U.S. Government Securities [Member]
Assets at Fair Value
Short-term investments		1,198
Level 1 [Member] \| Corporate Securities [Member]
Assets at Fair Value
Cash equivalents	0
Short-term investments	0	0
Level 1 [Member] \| Treasury Bills [Member]
Assets at Fair Value
Short-term investments		1,195
Level 2 [Member]
Assets at Fair Value
Total Assets at Fair Value	10,001	8,777
Level 2 [Member] \| Money Market Funds [Member]
Assets at Fair Value
Cash equivalents	0	0
Level 2 [Member] \| Commercial Paper [Member]
Assets at Fair Value
Cash equivalents	3,044	1,345
Short-term investments	5,110	6,776
Level 2 [Member] \| Treasury Bills [Member]
Assets at Fair Value
Short-term investments	0
Level 2 [Member] \| U.S. Government Securities [Member]
Assets at Fair Value
Short-term investments		0
Level 2 [Member] \| Corporate Securities [Member]
Assets at Fair Value
Cash equivalents	599
Short-term investments	1,248	656
Level 2 [Member] \| Treasury Bills [Member]
Assets at Fair Value
Short-term investments		0
Level 3 [Member]
Assets at Fair Value
Total Assets at Fair Value	0	0
Level 3 [Member] \| Money Market Funds [Member]
Assets at Fair Value
Cash equivalents	0	0
Level 3 [Member] \| Commercial Paper [Member]
Assets at Fair Value
Cash equivalents	0	0
Short-term investments	0	0
Level 3 [Member] \| Treasury Bills [Member]
Assets at Fair Value
Short-term investments	0
Level 3 [Member] \| U.S. Government Securities [Member]
Assets at Fair Value
Short-term investments		0
Level 3 [Member] \| Corporate Securities [Member]
Assets at Fair Value
Cash equivalents	0
Short-term investments	$ 0	0
Level 3 [Member] \| Treasury Bills [Member]
Assets at Fair Value
Short-term investments		$ 0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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)
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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), classified as current.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26626-111562

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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.

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