Leases - Operating Leases (Details) $ in Thousands	6 Months Ended
	Jun. 30, 2019 USD ($) ft² item	Jun. 30, 2018 USD ($)	Mar. 31, 2019 USD ($)	Feb. 29, 2016 ft²
Lease terms
Number of real estate leases | item	2
Lease, Practical Expedients, Package [true false]	true
Lessee, Operating Lease, Existence of Option to Extend [true false]	true
Weighted-average remaining non-cancelable lease term	7 years
Right-of-use assets	$ 16,389
Lease liabilities	26,095
Unamortized lease incentive and straight-line lease liability balances	$ 9,700
Weighted-average discount rate	11.00%
Aggregate adjustment to ROU asset	$ 1,000
Lease expense for operating lease payments	2,300	$ 2,800
Cash paid against operating lease liabilities	$ 2,600
830 Winter Street, Waltham, MA
Lease terms
Area of Real Estate Property | ft²	120,000
Number of additional terms for which lease agreement can be extended | item	2
Operating lease term extension period	5 years
930 Winter Street, Walham, MA
Lease terms
Area of Real Estate Property | ft²				10,281
ASU 2016-2 | Restatement Adjustment
Lease terms
Right-of-use assets			$ 17,600
Lease liabilities			$ 27,300