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
	Mar. 31, 2019	Mar. 31, 2018	Jan. 01, 2019	Dec. 31, 2018
Significant Accounting Policies [Line Items]
Current portion of deferred revenue	$ 7,000
Long-term portion of deferred revenue	12,272
Accumulated deficit	(138,286)			$ (122,828)	[1]
Collaboration revenues	1,750
Net loss	$ (17,670)	$ (12,954)
Net loss per share, basic and diluted	$ (0.41)	$ (1.37)
Balances Without the Adoption of Topic 606
Significant Accounting Policies [Line Items]
Current portion of deferred revenue	$ 6,250			6,250
Long-term portion of deferred revenue	15,421			16,984
Accumulated deficit				$ (122,828)
Collaboration revenues	1,563
Net loss	$ (17,857)
Net loss per share, basic and diluted	$ (0.41)
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
Long-term portion of deferred revenue	(3,149)
Accumulated deficit	(2,399)
Collaboration revenues	187
Net loss	$ 187

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