Common Stock (Details) $ / shares in Units, $ in Thousands		1 Months Ended		3 Months Ended
	Oct. 09, 2018 USD ($) $ / shares shares	May 31, 2019 $ / shares shares	Sep. 30, 2018	Sep. 30, 2019 shares	Jun. 30, 2019 USD ($)	Dec. 31, 2018 shares
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)				8,874,689		12,067,162
Stock split conversion rate			0.7378
Proceeds from IPO | $	$ 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 | $					$ 349,709
Stock options
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)				4,915,418		7,588,405
Restricted stock units
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)				443,267		0
2018 Incentive Award Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)				2,806,659		3,556,507
2018 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)				709,345		922,250
Convertible Preferred Stock
Class of Stock [Line Items]
Stock issued, conversion of convertible securities (in shares)	58,264,577
Common Stock
Class of Stock [Line Items]
Issuance of common stock upon follow-on offering, shares		5,175,000		949,496
Stock issued, conversion of convertible securities (in shares)	7,636
IPO
Class of Stock [Line Items]
Issuance of common stock upon follow-on offering, shares	14,375,000
Sale of stock, price per share | $ / shares	$ 19.00
Underwriters Over-Allotment Option | Common Stock
Class of Stock [Line Items]
Issuance of common stock upon follow-on offering, shares		675,000