Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2020	May 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	8,871,905		8,871,905		8,871,905		8,407,162
Issuance of common stock in public offering, net of offering costs (in shares)	4,312,500	5,175,000
Share price of stock issued (in usd per share)	$ 84.00	$ 71.00	$ 84.00		$ 84.00
Issuance of common stock in public offering, net of offering costs	$ 354,600	$ 349,700	$ 354,600	$ 349,709	$ 354,600	$ 349,709
Shares underlying outstanding stock options
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	3,869,002		3,869,002		3,869,002		4,494,889
Shares underlying unvested restricted stock units
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	564,008		564,008		564,008		496,131
Shares available for issuance under the 2018 Incentive Award Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	2,864,528		2,864,528		2,864,528		2,726,225
Shares available for issuance under the 2018 Employee Stock Purchase Plan
Class of Stock [Line Items]
Common stock, shares reserved for future issuance (in shares)	1,574,367		1,574,367		1,574,367		689,917