Leases - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Jul. 31, 2019 USD ($) ft²	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Oct. 31, 2017 ft²	Dec. 31, 2012 USD ($)
Operating Leased Assets [Line Items]
Right of use asset, net		$ 11,216		$ 11,216			$ 301
Lessee-paid construction costs		$ 632	$ 555	$ 2,094	$ 1,662
Operating lease, weighted average discount rate, percent		8.90%		8.90%			10.00%
Operating lease, expense		$ 600	$ 500	$ 2,000	$ 1,500
Lease liabilities		10,761		10,761			$ 372
75 North Beacon, Watertown, MA
Operating Leased Assets [Line Items]
Area of additional office space leased | ft²										5,100
Right of use asset, net								$ 0	$ 200
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Area of additional office space leased | ft²	25,078
Right of use asset, net						$ 11,800
Lessee-paid construction costs	$ 800
Term of contract	8 years
Operating lease, expense	$ 200
Lease liabilities						$ 11,800
Letter of credit
Operating Leased Assets [Line Items]
Restricted cash	$ 1,400	$ 300		$ 300
Maximum | Laboratory and office space
Operating Leased Assets [Line Items]
Leasehold improvements gross											$ 700