Organization and Description of Business - Additional Information (Details)			1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 14, 2019 USD ($) $ / shares shares	Apr. 01, 2018 shares	Nov. 30, 2018 USD ($)	Apr. 30, 2018 shares	Oct. 31, 2017 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Organization And Description Of Business [Line Items]
Operating losses						$ (10,413,000)	$ (4,707,000)	$ (23,887,000)	$ (21,772,000)	$ 29,400,000	$ 22,600,000
Accumulated deficit						(98,351,000)		(98,351,000)		(71,020,000)	(39,200,000)
Cash on hand						35,686,000		35,686,000		20,029,000	5,300,000
Outstanding borrowings from lenders						27,000,000		27,000,000		27,500,000	10,300,000
Available capacity on borrowings						800,000		800,000		1,400,000	$ 5,600,000
Equity financing						102,300,000		102,300,000		72,600,000
Repaying of term loan									$ 1,344,000
Term Loan
Organization And Description Of Business [Line Items]
Outstanding borrowings from lenders						$ 15,000,000		$ 15,000,000
Credit facility maximum borrowing amount										15,000,000
Term Loan | Midcap
Organization And Description Of Business [Line Items]
Credit facility maximum borrowing amount					$ 7,000,000
Horizon Term Loan
Organization And Description Of Business [Line Items]
Borrowings from term loan										15,000,000
Repaying of term loan										$ 4,900,000
Original Credit Facility | Midcap
Organization And Description Of Business [Line Items]
Credit facility maximum borrowing amount					15,000,000
Additional increase in borrowing amount					$ 30,000,000
Revolving Credit Facility | Midcap
Organization And Description Of Business [Line Items]
Credit facility maximum borrowing amount			$ 25,000,000
Additional increase in borrowing amount			$ 50,000,000
Initial Public Offering
Organization And Description Of Business [Line Items]
Common stock, shares issued | shares	3,600,000
Shares issued, price per share | $ / shares	$ 10.00
Net proceeds from sale of common stock	$ 29,600,000
Series C Holders
Organization And Description Of Business [Line Items]
Shares outstanding | shares		0.9
All Stock And Warrants Other Than Series C Preferred Stock
Organization And Description Of Business [Line Items]
Common stock, warrants and preferred stock to common stock conversion ratio		0.3131
Series C Preferred Stock
Organization And Description Of Business [Line Items]
Preferred stock to common stock, conversion ratio		0.2564
Common stock warrants to common stock conversion ratio		0.2564
Common stock, shares issued | shares				5,992,750