Fair Value of Financial Instruments - Additional Information (Detail) - USD ($)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Fair Value of Financial Instruments [Line Items]
Restricted cash	$ 527,000	$ 4,409,000
Fair value of note payable to related party, description	The fair value measurement of the note payable as of December 31, 2018 is based on discounted cash flow model and it is sensitive to the change in yield. If the yield changes by approximately 2%, the fair value of the note payable with the conversion feature would change approximately 3.8%. Similarly, if the yield changes by approximately 6%, the fair value of the note payable with the conversion feature would change approximately 11.1%. If the yield changes by approximately 8%, the fair value of the note payable with the conversion feature would change approximately 15.0%
Fair value of the liability, percentage decrease	6.00%
Minimum
Fair Value of Financial Instruments [Line Items]
Probabilities of meeting the milestones, amount	$ 50,000,000
Probabilities of meeting the milestones, percentage decrease	5.00%
Fair value of the liability, percentage decrease	13.00%
Maximum
Fair Value of Financial Instruments [Line Items]
Probabilities of meeting the milestones, amount	$ 200,000,000
Probabilities of meeting the milestones, percentage decrease	10.00%
Fair value of the liability, percentage decrease	25.00%
Note Payable Yield Changes 1
Fair Value of Financial Instruments [Line Items]
Note payable, percentage of interest rate change	2.00%
Change in fair value of note payable percentage due to change in interest rate percentage	3.80%
Note Payable Yield Changes 2
Fair Value of Financial Instruments [Line Items]
Note payable, percentage of interest rate change	6.00%
Change in fair value of note payable percentage due to change in interest rate percentage	11.10%
Note Payable Yield Changes 3
Fair Value of Financial Instruments [Line Items]
Note payable, percentage of interest rate change	8.00%
Change in fair value of note payable percentage due to change in interest rate percentage	15.00%
Money Market Funds
Fair Value of Financial Instruments [Line Items]
Cash equivalents	$ 71,700,000	48,400,000
Restricted cash	$ 500,000	$ 4,400,000

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Fair value of financial instruments.

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Fair value of liability percentage decrease if probabilities of meeting milestones percentage decrease.

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Fair value of note payable to related party description.

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Amount of 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Amount of cash restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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