Preferred and Common Stock Warrants - Schedule of Outstanding Warrants (Detail) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Number of warrants
Number of warrants, Outstanding and exercisable warrants, Beginning Balance (in shares)	5,146,400
Number of warrants, Conversion of warrant to purchase preferred shares to warrants for the purchase of common stock and adjusted for the Exchange Ratio and Reverse Stock Split (in shares)	(4,657,350)
Number of warrants, Issuance of warrants for the purchase of common stock (in shares)	6,035,000
Number of warrants, Exercised (in shares)	(33,846)
Number of warrants, Cancelled (in shares)	0
Number of warrants, Outstanding and exercisable warrants, Ending Balance (in shares)	6,490,204	5,146,400
Weighted Average Exercise Price
Weighted Average Exercise Price, Outstanding and exercisable warrants, Beginning Balance (in usd per share)	$ 0
Weighted Average Exercise Price, Issuance of warrants for the purchase of common stock (in usd per share)	12.43
Weighted Average Exercise Price, Exercised (in usd per share)	13.20
Weighted Average Exercise Price, Cancelled (in usd per share)	0
Weighted Average Exercise Price, Outstanding and exercisable warrants, Ending Balance (in usd per share)	$ 13.03	$ 0
Weighted Average Contractual Term (Years)	2 years 11 months 23 days	4 years 2 months 23 days