Leases (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) item	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 15, 2018	Mar. 15, 2018	Sep. 30, 2013 USD ($)
Leases
Restricted cash	$ 12,580	$ 2,178	$ 12,580	$ 2,178	$ 12,206
Remaining lease term			2 years 4 months 24 days
Number of vehicles leased under a master fleet lease agreement | item	65		65
Term of vehicle lease			3 years
Amount borrowed	$ 500		$ 500
Lease cost
Operating lease cost	1,185	99	3,428	296
Short-term lease cost		55		117
Total lease cost	1,185	154	3,428	413
Operating cash outflows from operating leases	$ 1,443	$ 103	$ 3,097	$ 307
Watertown Lease
Leases
Lease term	13 years		13 years
Restricted cash	$ 2,000		$ 2,000					$ 100
Discount rate (as a percent)						9.90%
Variable lease cost			$ 900
Remaining lease term			12 years 1 month 6 days
Waltham Lease
Leases
Lease term								3 years
Existence of option to extend			true
Waverley Oaks Lease
Leases
Lease term							1 year