Commitments and Contingencies (Details) € in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Feb. 28, 2017 EUR (€)	Apr. 30, 2013 ft²	Mar. 31, 2017 USD ($)	Sep. 30, 2017 GBP (£) ft² item	Sep. 30, 2017 EUR (€) ft²	Sep. 30, 2017 USD ($) ft²	Dec. 31, 2016 USD ($)	Feb. 29, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Apr. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Operating leases
Construction allowance						$ 5,325,000	$ 5,914,000
Minimum rental commitments under the non-cancelable operating lease agreements
2017 (three months remaining)						1,996,000
2018						7,763,000
2019						7,262,000
2020						7,311,000
2021						7,135,000
Thereafter						30,911,000
Total minimum lease payments						62,378,000
Obligations under capital leases						0
Collaborations and Licenses
Potential milestone payable						160,000,000
Obligation to make payment to vendor for certain contractual services | £				£ 300,000
Manufacturing Commitments
Collaborations and Licenses
Obligation to make payment to vendor for certain contractual services | €	€ 46.2
Other commitment						1,800,000
Due 2017						700,000
Due 2018						$ 1,100,000
Term of agreement	5 years
Noncancelable commitment | €					€ 13.6
830 Winter Street, Waltham, MA
Operating leases
Area of space leased | ft²				110,000	110,000	110,000
Number of additional terms for which lease agreement can be extended | item				2
Operating lease term extension period				5 years
Construction allowance									$ 186,000	$ 1,100,000	$ 746,000
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
Operating lease term extension period				5 years
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