Significant agreements - Additional Information (Detail) In Thousands, unless otherwise specified	0 Months Ended	3 Months Ended	9 Months Ended											1 Months Ended	0 Months Ended
	Mar. 19, 2013 Celgene Corporation [Member]	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement [Member] USD ($) Deliverables	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement [Member] USD ($) Option Deliverables	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement [Member] Maximum [Member]	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement [Member] Minimum [Member]	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Product [Member] Maximum [Member] Option Fees [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Product [Member] Maximum [Member] Clinical Milestone Payments [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Product [Member] Maximum [Member] Regulatory Milestone Payments [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Product [Member] Maximum [Member] Commercial Milestones Payments [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Co-promotion and Development [Member] Maximum [Member] Option Fees [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Co-promotion and Development [Member] Maximum [Member] Clinical Milestone Payments [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Co-promotion and Development [Member] Maximum [Member] Regulatory Milestone Payments [Member] USD ($)	Sep. 30, 2014 Celgene Corporation [Member] Collaborative Arrangement, Co-promotion and Development [Member] Maximum [Member] Commercial Milestones Payments [Member] USD ($)	Jan. 31, 2011 Association Francaise Contre Les Myopathies [Member] Research and Development Arrangement [Member] EUR (€)	Mar. 19, 2013 Up-front Payment Arrangement [Member] Celgene Corporation [Member] Collaborative Arrangement [Member] USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration agreement, cash payment received															$ 75,000
Term of collaboration agreement	3 years
Number of extension options			2
Maturity date of extension option				Mar. 19, 2019
Amount per product eligible to be received upon achievement of specified event						20,000	10,000	117,000	78,000	10,000	10,000	54,000	36,000
Collaboration period			3 years
Extended options period			2 years
Percentage of Change in control transaction which provides for the right to terminate agreement					50.00%
Number of deliverable		3	3
Deferred revenue recognition period			3 years
Deferred revenue recognized		6,250	18,750
Deferred revenue		$ 36,458	$ 36,458											€ 1,000
Term of research funding agreement														4 years