Leases - Lease Commitments (Details) $ in Millions		3 Months Ended		6 Months Ended
	Jan. 01, 2019 ft² item	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
Leases
Lease, Practical Expedients, Package	true
Lease, Practical Expedient, Use of Hindsight	false
Number of other non-significant operating leases	2
Lessee, Operating Lease, Existence of Option to Extend	true
Renewal term	3 years
Waltham, Massachusetts lease
Leases
Number of significant operating leases	1
Significant operating lease space | ft²	20,062
Operating lease expense | $		$ 0.2	$ 0.2	$ 0.3	$ 0.3