Commitments and Contingencies (Details) (USD $)	1 Months Ended	9 Months Ended	1 Months Ended
	Apr. 30, 2014 item	Mar. 31, 2015	Apr. 30, 2013 sqft	Dec. 31, 2013 sqft	Apr. 30, 2012 sqft	Dec. 31, 2009 sqft	Jul. 27, 2007 sqft
Minimum rental commitments under the non-cancelable operating lease agreements
2015 (three months remaining)		1,781,000
2016		6,926,000
2017		6,942,000
2018		7,048,000
2019		6,237,000
Thereafter		43,900,000
Total minimum lease payments		72,834,000
Total minimum rental payments from sublease		(395,000)
Total minimum lease payments, net		72,439,000
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)							89,000
Additional area of space leased (in square feet)				19,000
Construction allowance receivable	$ 1,100,000			$ 746,000
Number of additional terms for which lease agreement can be extended	2
Additional term period for which lease agreement can be extended	5 years
Area of property covered under sublease agreement (in square feet)					7,310	14,100
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)			7,507
Initial lease term period			5 years 2 months
Additional term period for which lease agreement can be extended			5 years