Leases - Operating Leases (Details) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($) ft² item lease	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($) ft² lease agreement
Lessee, Lease, Description [Line Items]
Number of real estate leases | lease	1
Right-of-use assets	$ 12,745		$ 14,072
Lease liabilities	$ 19,403
Weighted-average discount rate	11.00%
Lease expense for operating lease payments	$ 3,000	$ 3,000
Cash paid against operating lease liabilities	$ 4,100	4,100
Weighted average remaining term of the operating leases	4 years 6 months
Sublease income	$ 3,700	$ 1,600
Maximum
Lessee, Lease, Description [Line Items]
100 basis point change effect on ROU asset	$ 1,000
CRP/King 830 Winter L.L.C.
Lessee, Lease, Description [Line Items]
Area of space leased | ft²	120,000
Number of additional terms for which lease agreement can be extended | item	2
Operating lease term extension period	5 years
Number of executed sub-lease spaces | lease			4
Area of executed sublease space | ft²			65,000
PDM 930 Unit, LLC
Lessee, Lease, Description [Line Items]
Area of executed sublease space | ft²	10,281
Winter Street 930 Waltham MA
Lessee, Lease, Description [Line Items]
Number of executed sub-lease spaces | agreement			4
Area of executed sublease space | ft²			65,000
Minimum rental payments over the remaining term of the sublease	$ 14,000