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						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Jan. 01, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Current portion of deferred revenue	$ 7,000						$ 7,000
Long-term portion of deferred revenue	13,772						13,772
Accumulated deficit	(188,728)						(188,728)			$ (122,828)	[1]
Collaboration revenues	1,750						5,250
Net loss	$ (22,352)	$ (28,090)	$ (17,670)	$ (10,812)	$ (13,533)	$ (12,955)	$ (68,112)	$ (37,299)
Net loss per share, basic and diluted	$ (0.51)			$ (0.25)			$ (1.56)	$ (1.16)
Balances Without the Adoption of Topic 606
Significant Accounting Policies [Line Items]
Current portion of deferred revenue	$ 7,917						$ 7,917			6,250
Long-term portion of deferred revenue	15,574						15,574			16,984
Accumulated deficit										$ (122,828)
Collaboration revenues	1,618						4,743
Net loss	$ (22,484)						$ (68,619)
Net loss per share, basic and diluted	$ (0.51)						$ (1.57)
ASC 606
Significant Accounting Policies [Line Items]
Current portion of deferred revenue									$ 7,000
Long-term portion of deferred revenue									14,022
Accumulated deficit									$ (120,616)
ASC 606 | Effect of Adoption Higher/ (Lower)
Significant Accounting Policies [Line Items]
Current portion of deferred revenue	$ (917)						$ (917)
Long-term portion of deferred revenue	(1,802)						(1,802)
Accumulated deficit	(2,399)						(2,399)
Collaboration revenues	132						507
Net loss	$ 132						$ 507

[1] The Condensed Consolidated Balance Sheet as of December 31, 2018 has been derived from the audited financial statements as of that date.