Leases, Licensed Technology and Other Commitments (Details) ft² in Thousands, $ in Thousands	1 Months Ended				3 Months Ended		9 Months Ended
	Jul. 31, 2016 USD ($)	Aug. 31, 2015 ft²	Mar. 31, 2015 renewal_option	Sep. 30, 2013 ft²	Sep. 30, 2016 USD ($) ft² renewal_option	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) ft² renewal_option	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Jan. 31, 2013 ft² building
Operating Leased Assets [Line Items]
Future minimum annual rental payments under operating lease due, remainder of year					$ 3,700		$ 3,700
Future minimum annual rental payments under operating lease due, year two					$ 29,000		29,000
Percentage of leased space placed into service					44.00%
Property and equipment, gross					$ 355,256		355,256		$ 296,666
Facility lease obligation					552,919		552,919		443,092
Future minimum annual rental payments under operating lease due, year three					31,000		31,000
Future minimum annual rental payments under operating lease due, year four					31,500		31,500
Future minimum annual rental payments under operating lease due, year five					32,100		32,100
Future minimum annual rental payments under operating lease due, thereafter					$ 325,300		$ 325,300
Lease Agreements
Operating Leased Assets [Line Items]
Area of office and laboratory (in square feet) | ft²					100		100
Number of consecutive renewal options | renewal_option					2		2
Operating lease renewal options, lease term in years							5 years
Payments for early lease termination	$ 1,700
Operating lease, letters of credit outstanding					$ 1,400		$ 1,400
Future minimum annual rental payments under operating lease due, remainder of year					1,600		1,600
Future minimum annual rental payments under operating lease due, year two					3,500		3,500
Operating lease rent expense					2,700	$ 2,800	7,300	$ 7,300
Contingent rent expense					168,000	$ 191,000	537,000	$ 574,000
Binney Street, Cambridge, Massachusetts | Lease Agreements
Operating Leased Assets [Line Items]
Operating lease term			15 years
Operating lease renewal options, lease term in years			5 years
Operating lease, letters of credit outstanding					9,200		9,200
Future minimum annual rental payments under operating lease due, remainder of year					2,200		2,200
Future minimum annual rental payments under operating lease due, year two					25,500		25,500
Area of laboratory and office space under lease agreement (in square feet) | ft²				386						244
Number of units with all of the rentable space leased | building										1
Number of units available for operating lease | building										2
Additional area of office space (in square feet) | ft²				142
Number of renewal options | renewal_option			3
Future minimum annual rental payments under operating lease due, year three					31,000		31,000
Future minimum annual rental payments under operating lease due, year four					31,500		31,500
Future minimum annual rental payments under operating lease due, year five					32,100		32,100
Future minimum annual rental payments under operating lease due, year six					32,700		32,700
Future minimum annual rental payments under operating lease due, thereafter					292,600		292,600
Binney Street, Cambridge, Massachusetts | Sublease Agreement
Operating Leased Assets [Line Items]
Operating lease term		10 years
Future minimum annual rental payments under operating lease due, remainder of year					5,300		5,300
Future minimum annual rental payments under operating lease due, year two					10,700		10,700
Area of laboratory and office space under lease agreement (in square feet) | ft²		160
Future minimum annual rental payments under operating lease due, year three					10,900		10,900
Future minimum annual rental payments under operating lease due, year four					11,100		11,100
Future minimum annual rental payments under operating lease due, year five					11,300		11,300
Future minimum annual rental payments under operating lease due, thereafter					65,500		65,500
Sublease income					4,600		4,600
Facility costs including depreciation expense					1,800		1,800
Building
Operating Leased Assets [Line Items]
Property and equipment, gross					103,204		103,204		0
Building | Binney Street, Cambridge, Massachusetts
Operating Leased Assets [Line Items]
Leased space placed into service					103,200
Leasehold Improvements
Operating Leased Assets [Line Items]
Property and equipment, gross					37,163		37,163		23,609
Leasehold Improvements | Binney Street, Cambridge, Massachusetts
Operating Leased Assets [Line Items]
Leased space placed into service					13,300
Construction in progress
Operating Leased Assets [Line Items]
Property and equipment, gross					$ 182,742		$ 182,742		$ 246,669