Commitments and Contingencies (Details)	1 Months Ended	3 Months Ended
	Apr. 30, 2013 ft²	Sep. 30, 2016 GBP (£) ft² item	Sep. 30, 2016 USD ($) ft²	Sep. 26, 2016 ft²	Jun. 30, 2016 USD ($)	Feb. 29, 2016 USD ($) ft²	Dec. 31, 2015 USD ($)	Apr. 30, 2014 USD ($)	Dec. 31, 2013 USD ($)
Operating leases
Construction allowance			$ 6,110,000		$ 6,236,000
Minimum rental commitments under the non-cancelable operating lease agreements
2017 (nine months remaining)			5,955,000
2018			8,056,000
2019			7,258,000
2020			7,254,000
2021			7,302,000
Thereafter			34,252,000
Total minimum lease payments			70,077,000
Total minimum rental payments from sublease			(220,000)
Total minimum lease payments, net			69,857,000
Obligations under capital leases			0
Collaborations and Licenses
Potential milestone payable			162,000,000
Amount reimbursable by a third party			$ 1,400,000
Obligation to make payment to vendor for certain contractual services | £		£ 300,000
830 Winter Street, Waltham, MA
Operating leases
Area of space leased | ft²		110,000	110,000
Number of additional terms for which lease agreement can be extended | item		2
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		10,281
Construction allowance			$ 193,000			$ 617,000,000
333 Providence Hwy, Norwood MA
Operating leases
Lease term extension period		5 years
100 River Ridge Drive, Norwood, MA
Operating leases
Area of space leased | ft²	7,507
Lease term extension period	5 years
Lease term period	5 years 2 months