Common Stock (Details) $ / shares in Units, $ in Thousands			1 Months Ended		12 Months Ended
	Oct. 09, 2018 USD ($) $ / shares shares	Sep. 19, 2018	May 31, 2019 USD ($) $ / shares shares	Sep. 30, 2018	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 shares
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)					7,911,031	12,067,162
Stock split, conversion ratio		0.7378		0.7378
Issuance of common stock upon follow-on offering, shares			5,175,000
Proceeds from initial public offering and follow on offering | $	$ 249,500
Share price of stock issued (in usd per share) | $ / shares			$ 71.00
Issuance of common stock upon follow-on offering, net of offering costs | $			$ 349,700		$ 349,709
Stock options issued and outstanding
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)					4,494,889	7,588,405
Shares available for future stock option grants
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)					2,726,225	3,556,507
2018 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)					689,917	922,250
Convertible Preferred Stock
Class of Stock [Line Items]
Issuance of convertible preferred stock, net of issuance costs (in shares)	58,264,577
Common Stock
Class of Stock [Line Items]
Issuance of common stock upon follow-on offering, shares			5,175,000		5,175,000
Issuance of convertible preferred stock, net of issuance costs (in shares)	7,636
Additional Paid-in Capital
Class of Stock [Line Items]
Issuance of common stock upon follow-on offering, net of offering costs | $			$ 349,700		$ 349,709
IPO
Class of Stock [Line Items]
Issuance of common stock upon follow-on offering, shares	14,375,000
Sale of stock, price (usd per share) | $ / shares	$ 19.00
Proceeds from initial public offering and follow on offering | $	$ 249,500