Stock-Based Compensation - Summary of Stock Option Activity (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Number of Shares Outstanding, Beginning Balance	81,482
Number of Shares, Granted	58,994
Number of Shares, Cancelled/forfeited/expired	(25,835)
Number of Shares Outstanding, Ending Balance	114,641	81,482
Number of Shares, Vested and unvested expected to vest, Ending Balance	112,280
Weighted Average Exercise Price Per Share, Outstanding, Beginning Balance	$ 113.68
Weighted Average Exercise Price Per Share, Granted	2.96
Weighted Average Exercise Price Per Share, Cancelled/forfeited/expired	45.83
Weighted Average Exercise Price Per Share, Outstanding, Ending Balance	72.02	$ 113.68
Weighted Average Exercise Price Per Share, Vested and unvested expected to vest, Ending Balance	$ 73.23
Weighted Average Remaining Contractual Term in Years, Outstanding	8 years 9 months 14 days	8 years 9 months 18 days
Weighted Average Remaining Contractual Term in Years, Granted	7 years 5 months 4 days
Weighted Average Remaining Contractual Term in Years, Cancelled/forfeited/expired	8 years 8 months 4 days
Weighted Average Remaining Contractual Term in Years, Vested and unvested expected to vest	8 years 9 months 7 days