Summary of Significant Accounting Policies - Schedule of Restricted Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 1,994	$ 1,902	$ 364	$ 364
Restricted cash, current (included within prepaid expenses and other current assets)	264	0	$ 0	$ 0
Other Assets
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	1,730	1,902
Letter of credit security: Cambridge lease | Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	264	264
Letter of credit security: Waltham lease | Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	250	250
Letter of credit security: Vienna Austria lease | Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	336	94
Letter of credit security: Boston lease | Letter of Credit
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	1,144	1,144
Corporate credit card collateral
Restricted Cash and Cash Equivalents Items [Line Items]
Other assets	$ 0	$ 150