Commitments and Contingencies (Details) € in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($) ft²	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2018 EUR (€) ft²	Dec. 31, 2018 USD ($) ft²	Apr. 30, 2018 USD ($) ft²	Jan. 02, 2018 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2013 ft²
Operating leases
Construction allowance			$ 5,129,000			$ 4,675,000		$ 5,129,000	$ 2,000,000
Facilities rent expense, net of sublease income	$ 3,500,000	$ 7,700,000	$ 6,800,000	$ 6,500,000
Minimum rental commitments under the non-cancelable operating lease agreements
2019						5,498,000
2020						5,419,000
2021						5,257,000
2022						5,323,000
2023						5,450,000
Thereafter						12,336,000
Total minimum lease payments						39,283,000
Operating costs and real estate taxes per year		$ 3,000,000
Obligations under capital leases						0
Collaborations and Licenses
Potential milestone payable						80,000,000
Manufacturing commitments
Collaborations and Licenses
Manufacturing commitment						$ 1,300,000
Noncancelable commitment | €					€ 22
830 Winter Street, Waltham, MA
Operating leases
Area of space leased | ft²					120,000	120,000	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
333 Providence Hwy, Norwood MA
Operating leases
Area of space leased | ft²					43,850	43,850
100 River Ridge Drive, Norwood, MA
Operating leases
Area of space leased | ft²										7,507
Operating lease term period										5 years 2 months