Commitments and Contingencies (Details) € in Millions	1 Months Ended		3 Months Ended
	Feb. 28, 2017 EUR (€)	Apr. 30, 2013 ft²	Mar. 31, 2018 EUR (€) ft² item	Mar. 31, 2017 USD ($)	Apr. 30, 2018 USD ($) ft²	Mar. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Feb. 29, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Apr. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Operating leases
Construction allowance						$ 4,933,000	$ 5,129,000
Minimum rental commitments under the non-cancelable operating lease agreements
2018 (nine months remaining)						5,717,000
2019						7,202,000
2020						7,251,000
2021						7,074,000
2022						7,125,000
Thereafter						23,531,000
Total minimum lease payments						57,900,000
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						$ 2,600,000
Term of agreement	5 years
Noncancelable commitment | €			€ 13.9
830 Winter Street, Waltham, MA
Operating leases
Area of space leased | ft²			110,000		10,000	110,000
Number of additional terms for which lease agreement can be extended | item			2
Operating lease term extension period			5 years
Construction allowance					$ 400,000				$ 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
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