Organization and Description of Business (Details) $ / shares in Units, $ in Thousands			1 Months Ended			3 Months Ended	9 Months Ended
	Sep. 28, 2018 USD ($) $ / shares shares	Apr. 23, 2018	Sep. 30, 2018	Mar. 31, 2016 shares	May 31, 2015 shares	Sep. 30, 2018 shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Shares issued (in shares)					4
Shares exchange ratio (as a percent)		100
Proceeds from initial public offering, net of issuance costs paid in the period | $							$ 67,026
Risks and Uncertainties
Cash, cash equivalents and short-term investments | $								$ 48,800
Accumulated deficit | $								$ (119,917)	$ (90,109)
Initial Public Offering
Shares issued (in shares)	5,000,000
Shares exchange ratio (as a percent)			0.0482
Share price (in dollars per share) | $ / shares	$ 15.00
Proceeds from initial public offering, net of issuance costs paid in the period | $	$ 65,600
Preferred stock including accrued dividends, converted into common stock	8,084,414
Series A Redeemable Convertible Preferred Stock
Shares issued (in shares)					33,499,900
Preferred stock including accrued dividends, converted into common stock						(33,499,900)	(33,499,900)
Series B Redeemable Convertible Preferred Stock
Shares issued (in shares)				25,000,000
Preferred stock including accrued dividends, converted into common stock						(25,000,000)	(25,000,000)