Common Stock (Details)	1 Months Ended	3 Months Ended
	Sep. 30, 2017 USD ($)	Mar. 31, 2019 USD ($) Vote shares	Mar. 31, 2018 USD ($)	Dec. 31, 2018 shares
Common Stock
Common stock, shares authorized for issuance		290,000,000		290,000,000
Preferred stock, shares authorized for issuance		10,000,000		10,000,000
Number of votes per share of common stock | Vote		1
Dividends on common stock | $		$ 0
Proceeds from sale of stock | $			$ 64,877,000
Shares of common stock reserved for issuance, on an as-converted basis
Shares available for future option grants		3,391,001		2,486,637
Outstanding options		4,040,925		3,778,259
Unvested restricted common stock (founders and early exercise of stock options)		27,073		43,076
Shares reserved for employee stock purchase plan		400,000		400,000
Total		7,858,999		6,707,972
At-the-market offering | Cowen
Common Stock
Proceeds from sale of stock | $		$ 0
At-the-market offering | Cowen | Maximum
Common Stock
Potential gross proceeds | $	$ 125,000,000
Stock issuance costs, as a percent of gross proceeds	3.00%