Stockholders' Equity (Deficit) - Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands				2 Months Ended	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 26, 2019	May 20, 2019	Mar. 12, 2018	Feb. 28, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Sep. 22, 2019	Dec. 31, 2017
Number of shares issued				283,452
Purchase price (in dollar per share)				$ 6.88
Common stock, par value			$ 0.001		$ 0.001			$ 0.001		$ 0.001
Issuance costs paid					$ 1,991	$ 11	$ 5,898
Proceeds from issuance of common stock, net				$ 1,950				$ 17,736	$ 56,512
Underwriting discounts and commissions										$ 4,200
Deferred offering costs												$ 461
IPO
Number of shares issued			5,454,545
Purchase price (in dollar per share)			$ 11.00
Gross proceeds from issuance of common stock			$ 60,000
Proceeds from issuance of common stock, net			54,102
Underwriting discounts and commissions			4,200
Deferred offering costs			$ 1,698
Jefferies Sale Agreement
Number of shares issued					44,449			66,193
Common stock, par value		$ 0.001
Commission on gross sales proceeds (as a percent)		3.00%
Issuance costs paid					$ 390			$ 400
Gross proceeds from issuance of common stock					496			737
Proceeds from issuance of common stock, net					$ 106			$ 337
Deferred offering costs											$ 400
Jefferies Sale Agreement | Maximum
Proceeds from issuance of common stock, net		$ 20,000
September 2019 Offering
Number of shares issued	2,303,030
Purchase price (in dollar per share)	$ 8.25
Gross proceeds from issuance of common stock	$ 19,000
Proceeds from issuance of common stock, net	$ 17,399