Equity (Details) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2019 $ / shares shares	Jul. 20, 2018 USD ($) $ / shares shares	Jul. 31, 2018 shares	Sep. 30, 2018 shares	Sep. 30, 2019 Vote $ / shares shares	Sep. 30, 2018 USD ($)	Aug. 01, 2019 USD ($)	Dec. 31, 2018 $ / shares shares	Jun. 30, 2018 shares	Apr. 30, 2018 shares	Feb. 28, 2018 shares	Jan. 31, 2018 shares
Equity
Number of votes per share | Vote					1
Common stock, authorized (in shares)	150,000,000	150,000,000			150,000,000			150,000,000	79,000,000	78,800,000	75,000,000	65,000,000
Shares authorized		160,000,000
Common stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001			$ 0.001			$ 0.001
Preferred stock, share authorized (in shares)	10,000,000	10,000,000			10,000,000			10,000,000
Preferred stock, par value (in dollars per share) | $ / shares	$ 0.001	$ 0.001			$ 0.001			$ 0.001
Net proceeds after deducting underwriting discounts and commissions and other offering costs | $						$ 257,866
Common stock
Equity
Shares issued (in shares)				12,055,450
Conversion of redeemable convertible preferred stock to common stock (in shares)		51,845,438	51,845,438	51,845,438
IPO
Equity
Shares issued (in shares)		12,055,450
Net proceeds after deducting underwriting discounts and commissions and other offering costs | $		$ 254,300
Conversion of redeemable convertible preferred stock to common stock (in shares)			131,273
IPO | Common stock
Equity
Shares issued (in shares)		12,055,450
Overallotment
Equity
Shares issued (in shares)		1,572,450
At the market offering
Equity
Shares issued (in shares)	0
Percentage of commission							3.00%
Aggregate offering price | $							$ 100,000