Nature of the Business and Basis of Presentation - Additional Information (Details) $ / shares in Units, $ in Thousands								1 Months Ended			3 Months Ended								12 Months Ended
	Feb. 25, 2020 USD ($) $ / shares shares	Feb. 19, 2020 USD ($) $ / shares shares	Sep. 03, 2019 USD ($) $ / shares shares	Aug. 19, 2019 USD ($) $ / shares shares	Jun. 20, 2018 USD ($) $ / shares shares	Jun. 11, 2018 USD ($) $ / shares shares	Oct. 02, 2017 USD ($) $ / shares shares	Feb. 29, 2020 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Oct. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($) $ / shares shares	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Dec. 31, 2017 USD ($) shares
Nature Of Business And Basis Of Presentation [Line Items]
Share conversion ratio							5.65
Net loss											$ (67,216)	$ (56,196)	$ (21,460)	$ (47,384)	$ (32,299)	$ (24,435)	$ (21,690)	$ (21,430)	$ (192,256)	$ (99,854)	$ (50,284)
Accumulated deficit											(487,979)				$ (295,723)				(487,979)	$ (295,723)
Cash, cash equivalents and marketable securities											$ 579,600								$ 579,600
Common Shares
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares) | shares																			12,432,431	4,945,000	8,166,496
IPO
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares) | shares							7,500,000			8,166,496
Additional offering price of common stock (in dollars per share) | $ / shares										$ 17.00
Net proceeds from initial public offering							$ 114,100			$ 124,600
IPO | Common Shares
Nature Of Business And Basis Of Presentation [Line Items]
Additional offering price of common stock (in dollars per share) | $ / shares							$ 17.00
Follow On Offering | Common Shares
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares) | shares			1,621,621	10,810,810	645,000	4,300,000			4,945,000			12,432,431
Additional offering price of common stock (in dollars per share) | $ / shares			$ 37.00	$ 37.00	$ 40.00	$ 40.00			$ 40.00			$ 37.00					$ 40.00
Net proceeds from initial public offering			$ 56,400	$ 375,400	$ 24,300	$ 161,000			$ 185,300			$ 431,800
Follow On Offering | Common Shares | Subsequent Event
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock sold in public offering, net of underwriting discounts, commissions and offering costs (in shares) | shares	477,272	3,181,818						3,659,090
Additional offering price of common stock (in dollars per share) | $ / shares	$ 55.00	$ 55.00						$ 55.00
Net proceeds from initial public offering	$ 24,700	$ 163,700						$ 188,400