Leases and Other Commitments - Additional Information (Detail)	1 Months Ended			3 Months Ended		9 Months Ended
	Sep. 30, 2013 ft²	Jan. 31, 2013 ft²	May. 31, 2012 ft²	Sep. 30, 2015 USD ($) ft² RenewalOptions	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) ft² RenewalOptions	Sep. 30, 2014 USD ($)	Aug. 31, 2015 ft²	Dec. 31, 2014 USD ($)
Operating Lease Obligations [Line Items]
Facility lease obligation				$ 220,600,000		$ 220,600,000			$ 196,000,000
Aggregate base salaries payable to officers, due on 2015				2,300,000		2,300,000
Aggregate base salaries payable to officers, due on 2016				13,300,000		13,300,000
Aggregate base salaries payable to officers, due on 2017				8,300,000		8,300,000
Lease Agreements
Operating Lease Obligations [Line Items]
Future minimum annual rental payments under operating lease due, remainder of year				4,700,000		4,700,000
Future minimum annual rental payments under operating lease due, year two				18,100,000		18,100,000
Future minimum annual rental payments under operating lease due, year three				33,700,000		33,700,000
Future minimum annual rental payments under operating lease due, year four				38,100,000		38,100,000
Future minimum annual rental payments under operating lease due, year five				36,200,000		36,200,000
Future minimum annual rental payments under operating lease due, thereafter				361,700,000		361,700,000
Operating lease rent expense				2,800,000	$ 1,900,000	7,300,000	$ 5,700,000
Contingent rent expense				191,000	$ 167,000	$ 574,000	$ 519,000
Lease Agreements | Binney Street, Cambridge, Massachusetts
Operating Lease Obligations [Line Items]
Operating lease renewal options, lease term in years						5 years
Operating lease, letters of credit outstanding				9,200,000		$ 9,200,000
Additional square feet of office space | ft²	142,000
Future minimum annual rental payments under operating lease due, remainder of year				2,400,000		2,400,000
Future minimum annual rental payments under operating lease due, year two				8,700,000		8,700,000
Future minimum annual rental payments under operating lease due, year three				25,500,000		25,500,000
Future minimum annual rental payments under operating lease due, year four				31,000,000		31,000,000
Future minimum annual rental payments under operating lease due, year five				31,500,000		$ 31,500,000
Area of laboratory and office space under lease agreement | ft²	386,000	244,000
Period in which leased facility will be available for occupancy		2015-03
Construction of tenant improvement expected completion date						In the third quarter of 2016
Construction in progress				225,900,000		$ 225,900,000
Facility lease obligation				220,600,000		220,600,000
Current portion of long-term facility lease obligation				3,976,000		$ 3,976,000
Operating lease term						15 years
Number of renewal options | RenewalOptions						3
Future minimum annual rental payments under operating lease due, thereafter				$ 357,400,000		$ 357,400,000
Lease Agreements | Cambridge Massachusetts
Operating Lease Obligations [Line Items]
Area of office and laboratory in square foot | ft²				100,000		100,000
Lease expiration date			2017-08			2019-07
Number of consecutive renewal options | RenewalOptions				2		2
Operating lease renewal options, lease term in years						5 years
Operating lease, letters of credit outstanding				$ 1,400,000		$ 1,400,000
Additional square feet of office space | ft²			26,000
Future minimum annual rental payments under operating lease due, remainder of year				1,900,000		1,900,000
Future minimum annual rental payments under operating lease due, year two				7,500,000		7,500,000
Future minimum annual rental payments under operating lease due, year three				7,100,000		7,100,000
Future minimum annual rental payments under operating lease due, year four				6,100,000		6,100,000
Future minimum annual rental payments under operating lease due, year five				3,600,000		3,600,000
Lease Agreements | Lausanne Switzerland
Operating Lease Obligations [Line Items]
Future minimum annual rental payments under operating lease due, remainder of year				300,000		300,000
Future minimum annual rental payments under operating lease due, year two				1,100,000		1,100,000
Future minimum annual rental payments under operating lease due, year three				1,000,000		1,000,000
Future minimum annual rental payments under operating lease due, year four				1,100,000		1,100,000
Future minimum annual rental payments under operating lease due, year five				1,100,000		1,100,000
Area of laboratory and office space under lease agreement | ft²		22,000
Operating lease term		10 years
Future minimum annual rental payments under operating lease due, thereafter				4,400,000		4,400,000
Year in which leased facility will be available for occupancy		2014
Number of lease years before optional extension and early termination can be exercised		5 years
Sublease Agreement | Binney Street, Cambridge, Massachusetts
Operating Lease Obligations [Line Items]
Future minimum annual rental payments under operating lease due, year two				10,700,000		10,700,000
Future minimum annual rental payments under operating lease due, year three				10,900,000		10,900,000
Future minimum annual rental payments under operating lease due, year four				11,100,000		11,100,000
Future minimum annual rental payments under operating lease due, year five				11,300,000		$ 11,300,000
Area of laboratory and office space under lease agreement | ft²								160,000
Construction of tenant improvement expected completion date						In the third quarter of 2016
Operating lease term						10 years
Future minimum annual rental payments under operating lease due, thereafter				65,600,000		$ 65,600,000
Future minimum annual rental payments under operating lease due, next twelve months				$ 5,300,000		$ 5,300,000