Leases and Facility Lease Obligations - Additional Information (Detail) (Lease Agreements, USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		1 Months Ended	3 Months Ended	1 Months Ended
	May 31, 2012 Landsdowne Street, Cambridge, Massachusetts sqft	Mar. 31, 2013 Landsdowne Street, Cambridge, Massachusetts sqft RenewalOptions	Mar. 31, 2012 Landsdowne Street, Cambridge, Massachusetts	Jan. 31, 2013 Binney Street, Cambridge, Massachusetts sqft	Mar. 31, 2013 Binney Street, Cambridge, Massachusetts RenewalOptions	Jan. 31, 2013 Lausanne Switzerland sqft	Mar. 31, 2013 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		$ 1.0		$ 5.8
Operating lease term	3 years				15 years	10 years
Additional square feet of office space	26,000
Operating lease rent expense		1.5	1.2
Future minimum annual rental payments under operating lease due, year one		4.9			5.2		1.0
Future minimum annual rental payments under operating lease due, year two		6.7			6.3		1.1
Future minimum annual rental payments under operating lease due, year three		6.2			16.9		1.1
Future minimum annual rental payments under operating lease due, year four		5.5			19.3		1.1
Future minimum annual rental payments under operating lease due, year five		5.6			19.6		1.1
Future minimum annual rental payments under operating lease due, thereafter		9.0			220.3		5.5
Area of laboratory and office space under lease agreement				244,000		22,000
Year in which leased facility will be available for occupancy				2015		2014
Expiration date of options to expand into additional space, year and month				2013-06
Expiration date of rights to expand into additional space, year and month				2014-06
Number of renewal options					3
Number of lease years after optional extension					5 years
Construction in progress and facility lease obligation					$ 19.5
Number of lease years before optional extension and early termination can be exercised						5 years