Leases - Narrative (Details) $ in Thousands	1 Months Ended	3 Months Ended
	Jul. 31, 2019 USD ($) ft²	Mar. 31, 2020 USD ($)	Mar. 31, 2019 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		$ 11,847		$ 301
Lease liabilities		$ 11,865		$ 372
Operating lease, weighted average discount rate, percent		8.90%		10.00%
Lessee-paid construction costs		$ 673	$ 552
Operating lease, expense		600	500
operating lease payments due		$ 16,546		$ 375
75 North Beacon, Watertown, MA
Operating Leased Assets [Line Items]
Right of use asset, net			0		$ 200
Area of additional office space leased | ft²						5,100
65 Grove Street, Watertown,MA
Operating Leased Assets [Line Items]
Term of contract		8 years
Area of additional office space leased | ft²	25,078
Lessee-paid construction costs	$ 800
Operating lease, expense			$ 200
Letter of credit
Operating Leased Assets [Line Items]
Restricted cash	$ 1,400	$ 300
Maximum | Laboratory and office space
Operating Leased Assets [Line Items]
Leasehold improvements gross							$ 700