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Commitments and contingencies - Additional Information (Detail) (USD $)
In Thousands, unless otherwise specified
0 Months Ended 9 Months Ended 9 Months Ended 36 Months Ended 3 Months Ended
Jun. 09, 2014
sqft
Jun. 03, 2013
sqft
Sep. 30, 2014
Jun. 30, 2014
Commercial Milestones Payments [Member]
Jun. 30, 2014
Pregenen [Member]
Jun. 30, 2014
Pregenen [Member]
Preclinical milestones [Member]
Jun. 30, 2014
Pregenen [Member]
Clinical Milestone Payments [Member]
Jun. 30, 2014
Pregenen [Member]
Commercial Milestones Payments [Member]
Sep. 30, 2014
French Credit d'Impot Recherche Program [Member]
Dec. 31, 2012
French Credit d'Impot Recherche Program [Member]
Sep. 30, 2014
Rent Commencement Date [Member]
Sep. 30, 2014
First Anniversary of the Rent Commencement Date [Member]
Sep. 30, 2014
Second Anniversary of the Rent Commencement Date [Member]
Commitments And Contingencies Disclosure [Line Items]                          
Lease period   9 years                      
Lease building space   43,600                      
Monthly lease payments   $ 209                      
Rent abatement   209                      
Contribution from the landlord towards the initial build-out of the space 1,234 6,538                      
Option to extend this lease   5 years                      
Cash-collateralized irrevocable standby letter of credit   1,253                 1,044 835 627
Lease expiration date     Mar. 31, 2015                    
Additional lease building space 9,869                        
Additional monthly lease payments 47                        
Amendment effective date 2014-09                        
Eligible percentage of research expense monetized                 30.00%        
Aggregate reimbursement                   807      
Current assets                 699        
Additional future contingent cash payments       $ 99,900 $ 135,000 $ 15,000 $ 20,100 $ 99,900