Commitments and Contingencies (Details) € in Millions	1 Months Ended	3 Months Ended	9 Months Ended
	Feb. 28, 2017 EUR (€)	Mar. 31, 2017 USD ($)	Sep. 30, 2018 USD ($) item	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 EUR (€) ft²	Sep. 30, 2018 USD ($) ft²	Apr. 30, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Feb. 29, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Apr. 30, 2013 ft²
Operating leases
Construction allowance							$ 4,854,000		$ 5,129,000		$ 2,000,000
Minimum rental commitments under the non-cancelable operating lease agreements
2018 (three months remaining)							2,183,000
2019							7,995,000
2020							7,877,000
2021							7,716,000
2022							7,782,000
Thereafter							25,778,000
Total minimum lease payments							59,331,000
Obligations under capital leases							0
Collaborations and Licenses
Potential milestone payable							80,000,000
Manufacturing commitments
Collaborations and Licenses
Obligation to make payment to vendor for certain contractual services | €	€ 46.2
Manufacturing commitment							$ 2,200,000
Term of agreement	5 years
Noncancelable commitment | €						€ 14.1
Expensed for materials delivered			$ 6,300,000
Forecast | Manufacturing commitments
Collaborations and Licenses
Manufacturing commitment				$ 1,300,000	$ 900,000
830 Winter Street, Waltham, MA
Operating leases
Area of space leased | ft²						110,000	110,000	10,000
Number of additional terms for which lease agreement can be extended | item			2
Operating lease term extension period						5 years	5 years
Construction allowance								$ 400,000
930 Winter Street, Walham, MA
Operating leases
Area of space leased | ft²										10,281
Construction allowance										$ 617,000
100 River Ridge Drive, Norwood, MA
Operating leases
Area of space leased | ft²												7,507
Operating lease term extension period												5 years
Operating lease term period												5 years 2 months
Facilities rent expense, net of sublease income		$ 169,000