Summary of Significant Accounting Policies - Summary of Impact of Adoption of Topic 606 on Condensed Consolidated Balance Sheet and Statement of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Collaboration and license revenue									$ 15,000
Net loss	$ (16,598)	$ (22,352)	$ (28,090)	$ (17,670)	$ (12,295)	$ (10,812)	$ (13,533)	$ (12,954)	$ (84,710)	$ (49,594)
Net loss per share, basic and diluted									$ (1.93)	$ (1.43)
Balances Without the Adoption of Topic 606
Significant Accounting Policies [Line Items]
Collaboration and license revenue									$ 14,722
Net loss									$ (84,988)
Net loss per share, basic and diluted									$ (1.94)
ASC 606 | Effect of Adoption Higher/ (Lower)
Significant Accounting Policies [Line Items]
Collaboration and license revenue									$ 278
Net loss									$ 278
Net loss per share, basic and diluted									$ 0.01