Commitment and Contingencies - Additional Information (Details) $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended
	Apr. 30, 2020 USD ($) ft²	Sep. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2020 USD ($)	Oct. 31, 2018 ft²	Jan. 31, 2015 ft²
Lessee Lease Description [Line Items]
Operating lease right-of-use asset		$ 28,194	$ 28,194
Operating lease liability		33,314	33,314
Increase in operating lease liability			(1,915)
Operating lease cost		800	1,900
Sublease income		500	1,400
Tenant improvement allowance credits		100	200
Operating cash flows for operating leases		$ 600	$ 1,900
Operating lease, weighted average discount rate		7.10%	7.10%
Operating lease, weighted average remaining lease term		9 years 3 months 18 days	9 years 3 months 18 days
Capital lease obligations				$ 200
Rent expense				2,000
ASC 842
Lessee Lease Description [Line Items]
Operating lease right-of-use asset					$ 9,100
Operating lease liability					$ 11,500
Redwood City Lease | Restricted Cash
Lessee Lease Description [Line Items]
Letter of credit		$ 900	$ 900
Redwood City Lease | The 700 Building
Lessee Lease Description [Line Items]
Lessee operating lease expiration month and year			2023-04
Area of space leased | ft²							42,000
Lease expiration date	Dec. 31, 2030
Lessee, operating lease, option to extend			Option to extend the lease term for the 700 Building and the 300 Building together for an additional ten years after December 31, 2030.
Lessee, operating lease option to extend lease term	10 years
Percentage of increase in annual base rent	3.50%
Operating lease, rent description	The annual base rent for the lease of the 700 Building remains unchanged through April 30, 2023, and increases to $2.8 million for the 12 month period ending April 30, 2024, after which the annual base rent increases by approximately 3.5% in each subsequent year of the lease term.
Tenant improvement allowance			$ 3,400
Redwood City Lease | The 700 Building | ASC 842
Lessee Lease Description [Line Items]
Increase in operating lease right-of-use asset	$ 14,800
Increase in operating lease liability	14,800
Redwood City Lease | The 700 Building | Lease Term Until April 30, 2023
Lessee Lease Description [Line Items]
Annual base rent for lease	$ 2,800
Redwood City Lease | The 300 Building
Lessee Lease Description [Line Items]
Area of space leased | ft²	19,000
Lease beginning date	Dec. 15, 2020
Lease expiration date	Dec. 31, 2030
Lessee, operating lease, option to extend			Option to extend the lease term for the 700 Building and the 300 Building together for an additional ten years after December 31, 2030
Lessee, operating lease option to extend lease term	10 years
Percentage of increase in annual base rent	3.50%
Operating lease, rent description	The annual base rent for the lease of the 300 Building is $1.2 million until December 31, 2021, after which the annual base rent will increase by approximately 3.5% in each subsequent year of the lease term.
Lease initial term expiration date	Dec. 14, 2020
Percentage of sublease building	50.00%
Tenant improvement allowance			$ 3,300
Redwood City Lease | The 300 Building | ASC 842
Lessee Lease Description [Line Items]
Operating lease right-of-use asset	$ 6,400
Operating lease liability	9,000
Redwood City Lease | The 300 Building | Lease Term Expiring Until December 31, 2021
Lessee Lease Description [Line Items]
Annual base rent for lease	1,200
Redwood City Lease | The 300 Building | Lease Term Expiring on December 14, 2020
Lessee Lease Description [Line Items]
Annual base rent for lease	$ 500
Cambridge, Massachusetts
Lessee Lease Description [Line Items]
Lessee operating lease expiration month and year			2023-02
Area of space leased | ft²						22,000
Lessee, operating lease, option to extend			Option to extend the term through February 2028
Cambridge, Massachusetts | Restricted Cash
Lessee Lease Description [Line Items]
Letter of credit		$ 200	$ 200	$ 200