Commitment and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended	9 Months Ended
	Dec. 31, 2017 ft² Phase $ / ft²	Sep. 30, 2016	Sep. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Commitment And Contingencies [Line Items]
Rent expense, net			$ 400,000	$ 1,000,000.0
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit					$ 1,300,000	$ 1,300,000
Maximum
Commitment And Contingencies [Line Items]
Tenant improvement allowance					10,900,000
Bedford, Massachusetts
Commitment And Contingencies [Line Items]
Operating lease beginning year and month		2016-11
Operating lease expiration year and month	2027-02	2021-10
Operating lease agreements additional term		3 years
Sublease aggregate base rent obligation					$ 2,700,000
Office space leased | ft²	67,000
Lessee, operating lease, lease not yet commenced, renewal term	5 years
Number of phases | Phase	2
Initial annual base rent per square foot | $ / ft²	39.50
Percentage increase in initial annual base rent per square foot.	3.00%
Bedford, Massachusetts | Phase One
Commitment And Contingencies [Line Items]
Office space leased | ft²	46,000
Rent due date	2018-09
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000
Rent due date	2019-03