Leases, Licensed Technology and Other Commitments - Additional Information (Detail) $ in Millions	1 Months Ended			12 Months Ended
	Sep. 30, 2013 ft²	Jan. 31, 2013 ft²	May. 31, 2012 ft²	Dec. 31, 2015 USD ($) ft² RenewalOptions	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Aug. 31, 2015 ft²
Operating Lease Obligations [Line Items]
Facility lease obligation				$ 231.7	$ 196.0
Aggregate base salaries payable to officers, due on 2016				15.8
Aggregate base salaries payable to officers, due on 2017				2.5
Aggregate base salaries payable to officers, due on 2018				0.7
Lease Agreements
Operating Lease Obligations [Line Items]
Future minimum annual rental payments under operating lease due, next twelve months				17.5
Future minimum annual rental payments under operating lease due, year two				32.6
Future minimum annual rental payments under operating lease due, year three				38.1
Future minimum annual rental payments under operating lease due, year four				36.2
Future minimum annual rental payments under operating lease due, year five				33.1
Future minimum annual rental payments under operating lease due, thereafter				328.5
Operating lease rent expense				9.6	7.6	$ 6.1
Contingent rent expense				$ 0.8	0.7	0.7
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				$ 9.2
Operating lease term				15 years
Additional square feet of office space | ft²	142,000
Future minimum annual rental payments under operating lease due, next twelve months				$ 8.7
Future minimum annual rental payments under operating lease due, year two				25.5
Future minimum annual rental payments under operating lease due, year three				31.0
Future minimum annual rental payments under operating lease due, year four				31.5
Future minimum annual rental payments under operating lease due, year five				$ 32.1
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 second half of 2016
Construction in progress				$ 246.7
Facility lease obligation				231.7
Future minimum annual rental payments under operating lease due, thereafter				$ 325.3
Number of renewal options | RenewalOptions				3
Lease Agreements | Lausanne Switzerland
Operating Lease Obligations [Line Items]
Operating lease term		10 years
Future minimum annual rental payments under operating lease due, year two				$ 1.0
Future minimum annual rental payments under operating lease due, year three				1.0
Future minimum annual rental payments under operating lease due, year four				1.0
Future minimum annual rental payments under operating lease due, year five				1.1
Area of laboratory and office space under lease agreement | ft²		22,000
Future minimum annual rental payments under operating lease due, thereafter				3.2
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
Future minimum annual rental payments under operating lease due, remainder of year				$ 1.0
Lease Agreements | Landsdowne and Sidney Streets, Cambridge Massachusetts
Operating Lease Obligations [Line Items]
Area of office and laboratory in square foot | ft²				100,000
Extended lease expiration date				2019-07
Number of consecutive renewal options | RenewalOptions				2
Operating lease renewal options, lease term in years				5 years
Operating lease, letters of credit outstanding				$ 1.4
Operating lease term			3 years
Additional square feet of office space | ft²			26,000
Future minimum annual rental payments under operating lease due, next twelve months				7.0
Future minimum annual rental payments under operating lease due, year two				6.0
Future minimum annual rental payments under operating lease due, year three				6.1
Future minimum annual rental payments under operating lease due, year four				3.6
Future minimum annual rental payments under operating lease due, year five				$ 0.0
Sublease Agreement | Binney Street, Cambridge, Massachusetts
Operating Lease Obligations [Line Items]
Operating lease term				10 years
Future minimum annual rental payments under operating lease due, next twelve months				$ 5.3
Future minimum annual rental payments under operating lease due, year two				10.7
Future minimum annual rental payments under operating lease due, year three				10.9
Future minimum annual rental payments under operating lease due, year four				11.1
Future minimum annual rental payments under operating lease due, year five				$ 11.3
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
Future minimum annual rental payments under operating lease due, thereafter				$ 65.6
Licensed Technology
Operating Lease Obligations [Line Items]
License fees paid				0.5	$ 0.5	$ 0.1
Licensed Technology | Year 2016
Operating Lease Obligations [Line Items]
Future expected annual license fees				0.8
Licensed Technology | Year 2017
Operating Lease Obligations [Line Items]
Future expected annual license fees				2.9
Licensed Technology | Year 2018
Operating Lease Obligations [Line Items]
Future expected annual license fees				0.1
Licensed Technology | Year 2019
Operating Lease Obligations [Line Items]
Future expected annual license fees				0.1
Licensed Technology | Year 2020
Operating Lease Obligations [Line Items]
Future expected annual license fees				$ 0.1