Commitments and Contingencies - Lease Commitments (Details)		3 Months Ended			6 Months Ended
	Mar. 31, 2017 USD ($) ft²	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($) ft²	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Lease Commitments
Restricted cash		$ 7,057,000			$ 7,057,000		$ 8,247,000
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		$ 4,500,000		$ 2,000	$ 5,500,000	$ 7,500,000