Leases - Operating Leases - Binney Street Lease (Details) $ in Thousands			3 Months Ended		6 Months Ended
	Jun. 11, 2019 USD ($) ft²	Apr. 01, 2019 ft²	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
Operating leases
Weighted-average discount rate of operating leases (as a percent)			5.80%	5.30%	5.80%	5.30%
Operating lease cost			$ 633	$ 3,155	$ 1,266	$ 7,680
Sublease income				300		300
Binney Street Lease
Operating leases
Rentable area leased (in square feet) | ft²	108,000	108,000
Surrendered space (in square feet) | ft²		114,000
Weighted-average discount rate of operating leases (as a percent)	4.00%	5.10%
Gain on modification of lease				3,200		3,200
Lease termination cost	$ 9,000
Operating lease cost				3,600		8,000
Sublease income				$ 300		$ 300