Leases and Facility Lease Obligation - Additional Information (Detail) (Lease Agreements, USD $) In Millions, unless otherwise specified	1 Months Ended	3 Months Ended		6 Months Ended		1 Months Ended	6 Months Ended	1 Months Ended
	May 31, 2012 Landsdowne and Sidney Streets, Cambridge Massachusetts sqft	Jun. 30, 2013 Landsdowne and Sidney Streets, Cambridge Massachusetts RenewalOptions sqft	Jun. 30, 2012 Landsdowne and Sidney Streets, Cambridge Massachusetts	Jun. 30, 2013 Landsdowne and Sidney Streets, Cambridge Massachusetts RenewalOptions sqft	Jun. 30, 2012 Landsdowne and Sidney Streets, Cambridge Massachusetts	Jan. 31, 2013 Binney Street, Cambridge, Massachusetts sqft	Jun. 30, 2013 Binney Street, Cambridge, Massachusetts RenewalOptions	Jul. 31, 2013 Binney Street, Cambridge, Massachusetts Subsequent Event sqft	Jan. 31, 2013 Lausanne Switzerland sqft	Jun. 30, 2013 Lausanne Switzerland
Operating Lease Obligations [Line Items]
Area of office and laboratory in square foot		100,000		100,000
Extended lease expiration date				2019-07
Number of consecutive renewal options		2		2
Operating lease renewal options, lease term in years				5 years
Operating lease, letters of credit outstanding		$ 1.0		$ 1.0		$ 5.8
Operating lease term	3 years						15 years		10 years
Additional square feet of office space	26,000							140,000
Operating lease rent expense		1.5	1.4	2.9	2.7
Future minimum annual rental payments under operating lease due, year one		3.3		3.3			5.2			1.0
Future minimum annual rental payments under operating lease due, year two		6.7		6.7			6.3			1.1
Future minimum annual rental payments under operating lease due, year three		6.2		6.2			16.9			1.1
Future minimum annual rental payments under operating lease due, year four		5.5		5.5			19.3			1.1
Future minimum annual rental payments under operating lease due, year five		5.6		5.6			19.6			1.1
Future minimum annual rental payments under operating lease due, thereafter		9.0		9.0			220.3			5.6
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							$ 46.5
Number of lease years before optional extension and early termination can be exercised									5 years