Common Stock (Details) $ / shares in Units, $ in Thousands		1 Months Ended	3 Months Ended
	Aug. 09, 2018 USD ($) $ / shares shares	Sep. 30, 2018	Jun. 30, 2019 USD ($) shares	Jun. 30, 2018 shares	May 31, 2019 $ / shares	Dec. 31, 2018 shares
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)			9,980,595			12,067,162
Stock split conversion rate		0.7378
Proceeds from IPO | $	$ 249,500
Share price of stock issued (in dollars 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)			5,677,472			7,588,405
Restricted stock units
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)			142,217			0
2018 Incentive Award Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)			3,358,358			3,556,507
2018 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)			802,548			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
Stock issued, conversion of convertible securities (in shares)	7,636
Issuance of common stock upon follow-on offering, net of offering costs | $			$ 349,709
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