Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Property and equipment, gross	$ 355,256		$ 355,256		$ 296,666
Facility lease obligation	$ 552,919		552,919		443,092
Percentage of leased space placed into service	44.00%
Depreciation and amortization expense	$ 1,900	$ 1,000	4,300	$ 2,700
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment, gross	182,742		182,742		246,669
Building
Property, Plant and Equipment [Line Items]
Property and equipment, gross	103,204		103,204		0
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment, gross	37,163		$ 37,163		$ 23,609
Binney Street, Cambridge, Massachusetts | Building
Property, Plant and Equipment [Line Items]
Leased space placed into service	103,200
Binney Street, Cambridge, Massachusetts | Leasehold Improvements
Property, Plant and Equipment [Line Items]
Leased space placed into service	$ 13,300