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				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 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	10,522				10,522
Accumulated deficit	(166,376)				(166,376)			$ (122,828)	[1]
Collaboration revenues	1,750				3,500
Net loss	$ (28,090)	$ (17,670)	$ (13,533)	$ (12,955)	$ (45,760)	$ (26,488)
Net loss per share, basic and diluted	$ (0.64)		$ (0.32)		$ (1.05)	$ (1.01)
Balances Without the Adoption of Topic 606
Significant Accounting Policies [Line Items]
Current portion of deferred revenue	$ 6,250				$ 6,250			6,250
Long-term portion of deferred revenue	13,859				13,859			16,984
Accumulated deficit								$ (122,828)
Collaboration revenues	1,563				3,126
Net loss	$ (28,277)				$ (46,134)
Net loss per share, basic and diluted	$ (0.65)				$ (1.06)
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	$ 750				$ 750
Long-term portion of deferred revenue	(3,337)				(3,337)
Accumulated deficit	(2,399)				(2,399)
Collaboration revenues	187				374
Net loss	$ 187				$ 374

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