Commitments and Contingencies (Details)	1 Months Ended		3 Months Ended
	Apr. 30, 2014 USD ($) item	Apr. 30, 2013 ft²	Sep. 30, 2015 USD ($)	Dec. 31, 2013 USD ($) ft²	Jul. 27, 2007 ft²
Minimum rental commitments under the non-cancelable operating lease agreements
2016 (nine months remaining)			$ 5,519,000
2017			7,147,000
2018			7,253,000
2019			6,441,000
2020			6,423,000
Thereafter			38,857,000
Total minimum lease payments			71,640,000
Total minimum rental payments from sublease			(340,000)
Total minimum lease payments, net			71,300,000
Obligations under capital leases			0
Collaborations
Potential milestone payable			162,000,000
Amount reimbursable by a third party			$ 1,400,000
830 Winter Street, Waltham, MA
Operating leases
Area of space leased (in square feet) | ft²					89,000
Additional area of space leased (in square feet) | ft²				19,000
Construction allowance receivable	$ 1,100,000			$ 746,000
Number of additional terms for which lease agreement can be extended | item	2
Additional term period for which lease agreement can be extended	5 years
333 Providence Hwy, Norwood MA
Operating leases
Additional term period for which lease agreement can be extended			5 years
100 River Ridge Drive, Norwood, MA
Operating leases
Area of space leased (in square feet) | ft²		7,507
Initial lease term period		5 years 2 months
Additional term period for which lease agreement can be extended		5 years