Leases - Narrative (Details) $ in Thousands		1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 01, 2019 USD ($)	Jul. 31, 2019 USD ($) ft²	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2017 ft²	Dec. 31, 2012 USD ($)
Operating Leased Assets [Line Items]
Right of use asset, net				$ 301
Lease liabilities				372
Operating lease, remaining lease term	15 months
Discount rate	10.00%
Lessee-paid construction costs				2,209
Operating lease, expense				2,100	$ 2,000	$ 1,900
operating lease payments due				$ 375
75 North Beacon, Watertown, MA
Operating Leased Assets [Line Items]
Right of use asset, net	$ 200		$ 0
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
operating lease payments due		15,800
Letter of credit
Operating Leased Assets [Line Items]
Restricted cash		$ 1,400		$ 300
Minimum
Operating Leased Assets [Line Items]
Term of contract				2 years
Renewal term				1 year
Maximum
Operating Leased Assets [Line Items]
Term of contract				8 years
Renewal term				5 years
Maximum | Laboratory and office space
Operating Leased Assets [Line Items]
Leasehold improvements gross								$ 700
Accounting Standards Update 2016-02
Operating Leased Assets [Line Items]
Right of use asset, net	1,600
Lease liabilities	1,800
Deferred rent and incentives	$ 200