Leases - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2019 USD ($) ft²	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 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		$ 11,474		$ 11,474			$ 301
Lessee-paid construction costs		$ 790	$ 555	$ 1,463	$ 1,107
Operating lease, weighted average discount rate, percent		8.90%		8.90%			10.00%
Operating lease, expense		$ 800	600	$ 1,400	1,100
Lease liabilities		$ 11,768		$ 11,768			$ 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		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		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