Leases - Lease Commitments (Details) $ in Millions		3 Months Ended		9 Months Ended
	Jan. 01, 2019 ft² item	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Operating 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
Operating leases
Number of significant operating leases	1
Significant operating lease space | ft²	20,062
Operating lease expense | $		$ 0.2	$ 0.2	$ 0.5	$ 0.5