Derivative Liability - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands				12 Months Ended
	Mar. 28, 2017	May 03, 2016	Jan. 29, 2016	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross amount				$ 0	$ 7,375
Cancellation of derivative liability				0	(3,036)
Net proceeds on issuance stock				$ 48,792	23,964
Common Stock
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Issuance of common stock, shares				5,250,000
Common Stock | Public Offering
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Issuance of common stock, shares	5,250,000
Net proceeds on issuance stock	$ 48,800
Share price (usd per share)	$ 10
Series AB Preferred Stock Warrant
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of warrants to acquire shares of common stock, shares			16,000,000
Exercise price (usd per share)			$ 1.25
Term of year from the date of issuance			1 year
Series AB Preferred Stock Warrant | CRG
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Gross amount					7,400
Fair value of exercised warrants					$ 1,600
Cancellation of derivative liability		$ 3,000
Series AB Preferred Stock Warrant | CRG | Warrants
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Issuance of common stock, shares					5,900,000
Series AB Preferred Stock Warrant | CRG | Common Stock
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Number of shares issued upon conversion, shares					175,938
Placement Agent Warrants
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Warrants issued, shares					10,390
Warrant term					5 years
Fair value of the warrant		$ 300		$ 0	$ 200