April 2014 Credit Facility - Additional Information (Detail) - USD ($)							12 Months Ended
	Sep. 20, 2018	Aug. 13, 2018	Jul. 01, 2018	Jan. 30, 2018	Mar. 31, 2017	Apr. 30, 2014	Dec. 31, 2014	Jun. 30, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Exercise price of unregistered warrants		$ 4.53		$ 4.53	$ 75.00
Warrant term					5 years
Issuance of unregistered warrants to purchase shares of common stock, grant date fair value	$ 2,000,000	$ 8,400,000		$ 9,700,000	$ 2,800,000
Remaining outstanding principal payment									$ 1,168,811
Oxford Finance LLC [Member] | Common Stock [Member]
Line of Credit Facility [Line Items]
Warrant issued to lender						588
Exercise price of unregistered warrants						$ 424.80
Warrant term						10 years
Oxford Finance LLC [Member] | First Term Loan [Member]
Line of Credit Facility [Line Items]
Net cash proceeds on term loan						$ 4,898,000
Line of Credit Facility fees amount payable						$ 50,000
Line of Credit Facility, interest rate during period						7.95%
Percentage of final interest payment due at maturity						5.50%
Issuance costs							$ 102,000
Net proceeds from credit facility							4,898,000
Issuance of unregistered warrants to purchase shares of common stock, grant date fair value							$ 233,000
Unamortized discount									$ 33,000
Effective annual interest rate								13.87%	13.87%
Remaining outstanding principal payment								$ 175,000
Total remaining principal payment paid			$ 175,000