Commitments and Contingencies - Lease Commitments (Details)		3 Months Ended
	Mar. 31, 2017 USD ($) ft²	Mar. 31, 2017 USD ($) ft²	Mar. 31, 2016 USD ($)
Lease Commitments
Restricted cash	$ 1,190,000	$ 1,190,000
(Gain) loss on facility subleases		$ (1,579,000)	$ 3,480,000
Office and laboratory space in Cambridge, Massachusetts
Lease Commitments
Rentable area leased (in square feet) | ft²	223,000	223,000
Rentable area subleased (in square feet) | ft²	93,000	93,000
Option to extend lease term	5 years
Annual rent escalation	$ 3	$ 3
Restricted cash	$ 6,400,000	6,400,000
(Gain) loss on facility subleases		1,600,000
Rent expense		$ 1,000,000	$ 5,500,000