Leases and Facility Lease Obligation - Additional Information (Detail) (USD $)		3 Months Ended		9 Months Ended		1 Months Ended		9 Months Ended	1 Months Ended	9 Months Ended	1 Months Ended
	Sep. 30, 2014	Sep. 30, 2014 Lease Agreements	Sep. 30, 2013 Lease Agreements	Sep. 30, 2014 Lease Agreements	Sep. 30, 2013 Lease Agreements	Sep. 30, 2013 Lease Agreements Binney Street, Cambridge, Massachusetts sqft	Jan. 31, 2013 Lease Agreements Binney Street, Cambridge, Massachusetts sqft	Sep. 30, 2014 Lease Agreements Binney Street, Cambridge, Massachusetts sqft RenewalOptions	May 31, 2012 Lease Agreements Landsdowne and Sidney Streets, Cambridge Massachusetts sqft	Sep. 30, 2014 Lease Agreements Landsdowne and Sidney Streets, Cambridge Massachusetts RenewalOptions sqft	Jan. 31, 2013 Lease Agreements Lausanne Switzerland sqft	Sep. 30, 2014 Lease Agreements Lausanne Switzerland
Operating Lease Obligations [Line Items]
Area of office and laboratory in square foot										100,000
Extended lease expiration date										2019-07
Number of consecutive renewal options										2
Operating lease renewal options, lease term in years										5 years
Operating lease, letters of credit outstanding								$ 9,200,000		$ 1,400,000
Operating lease term								15 years	3 years		10 years
Additional square feet of office space						142,000			26,000
Future minimum annual rental payments under operating lease due, remainder of year		2,300,000		2,300,000				5,300,000		1,800,000		261,000
Future minimum annual rental payments under operating lease due, year two		14,000,000		14,000,000				8,400,000		6,700,000		1,100,000
Future minimum annual rental payments under operating lease due, year three		15,600,000		15,600,000				26,600,000		5,900,000		1,100,000
Future minimum annual rental payments under operating lease due, year four		33,700,000		33,700,000				30,400,000		6,000,000		1,100,000
Future minimum annual rental payments under operating lease due, year five		37,600,000		37,600,000				30,900,000		6,100,000		1,100,000
Future minimum annual rental payments under operating lease due, thereafter		387,200,000		387,200,000				347,100,000		3,600,000		5,500,000
Area of laboratory and office space under lease agreement						386,000	244,000				22,000
Year in which leased facility will be available for occupancy							2015				2014
Construction of tenant improvement expected completion date								In late 2015 or early 2016
Construction in progress and facility lease obligation	171,300,000							171,300,000
Current portion of long-term facility lease obligation	4,971,000							5,000,000
Number of renewal options								3
Number of lease years after optional extension								5 years
Area expected to be subleased								170,000
Number of lease years before optional extension and early termination can be exercised											5 years
Operating lease rent expense		1,900,000	5,700,000	1,500,000	4,400,000
Contingent rent expense		$ 167,000	$ 519,000	$ 186,000	$ 526,000