Common Stock Warrants Outstanding - Summary of Equity-Classified Common Stock Warrant Activity (Detail) - Warrants [Member] - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Number of Shares, Outstanding, Beginning Balance	288,196
Number of Shares, Issued	4,406,731
Number of Shares, Outstanding, Ending Balance	4,814,927	288,196
Weighted Average Exercise Price Per Share, Outstanding, Beginning Balance	$ 78.86
Weighted Average Exercise Price Per Share, Issued	3.95
Weighted Average Exercise Price Per Share, Outstanding, Ending Balance	$ 5.44	$ 78.86
Average Remaining Contractual Term (in years)	4 years 8 months 12 days	4 years