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Stock Based Compensation - Additional Information (Detail) - USD ($)
1 Months Ended 12 Months Ended
Dec. 31, 2017
Jun. 30, 2017
Mar. 31, 2017
Sep. 30, 2016
Jun. 30, 2016
Feb. 29, 2016
Oct. 31, 2015
Sep. 30, 2015
Jun. 30, 2015
Jun. 30, 2013
Nov. 30, 2012
Aug. 31, 2012
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Term of award                         10 years          
Share based compensation granted under plan vest period                         4 years          
Weighted-average grant date fair value per share of stock-based awards, granted to employees                         $ 14.78 $ 12.46 $ 14.98      
Stock option grants with performance conditions                         4,805,722 [1] 1,285,051 2,830,078      
Stock-based compensation expense                         $ 30,465,000 $ 29,962,000 $ 32,117,000      
Former Chief Executive Officer [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock-based compensation expense                         $ 2,100,000   8,600,000      
Stock Options [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Share based compensation granted under plan vest period           4 years       4 years                
Stock option grants with performance conditions           300,000       400,000                
Forfeited awards percentage                         20.00%          
Stock-based compensation expense                         $ 23,416,000 26,320,000 29,014,000      
Stock Options [Member] | EXONDYS 51 [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Vesting percentage                               80.00%    
Stock Options [Member] | Performance Grants Vested Immediately [Member] | EXONDYS 51 [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Vesting percentage           50.00%                        
Stock Options [Member] | Vesting Subject to the Remaining Service Conditions [Member] | EXONDYS 51 [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Vesting percentage           50.00%                        
Stock Options with Performance-based Conditions [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock-based compensation expense                         900,000 $ 5,000,000 $ 500,000      
Service-Vesting Conditions [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock based compensation expense not yet recognized $ 47,500,000                       47,500,000          
Stock Options with Service-and Performance-Based Conditions [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock based compensation expense not yet recognized $ 400,000                       $ 400,000          
Restricted Stock Awards (RSAs) [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards                         341,500 117,553 181,783      
Grant date fair value, Restricted stock units and awards                         $ 34.58 $ 41.22 $ 20.80      
Restricted Stock Awards (RSAs) [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards               65,000                    
Grant date fair value, Restricted stock units and awards $ 54.29                                  
Restricted Stock Awards (RSAs) [Member] | Minimum [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards       71,925                            
Share-based compensation award, expiration date Jun. 30, 2018                                  
Restricted Stock Awards (RSAs) [Member] | Maximum [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards       89,906                            
Share-based compensation award, expiration date Dec. 31, 2018                                  
Stock-based compensation related to grants       $ 4,900,000                            
Restricted Stock Units (RSUs) [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards                         181,029          
Grant date fair value, Restricted stock units and awards                         $ 33.03          
Restricted Stock Units (RSUs) [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards     156,029                              
Restricted Stock Units (RSUs) [Member] | Maximum [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock-based compensation related to grants $ 700,000                       $ 700,000          
Restricted Stock Units (RSUs) [Member] | Performance Grants Vested Immediately [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock-based compensation expense                         2,500,000          
Percentage of RSUs vested upon achievement of first condition   50.00%                                
Restricted Stock Units (RSUs) [Member] | Vesting upon Achievement of Second Milestone [Member] | Executives [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Stock-based compensation expense                         400,000          
Stock-based compensation related to grants $ 300,000                       $ 300,000          
Stock Appreciation Rights [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Grants, Restricted stock units and awards                     100,000 70,000            
Exercise price of SARs                     $ 23.85 $ 10.08            
2011 Equity Incentive Plan [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Common stock remain available for future grant 2,300,000                       2,300,000          
Number of shares approved under the plan                                   13,000,000
Number of additional shares approved under the plan         1,300,000       1,700,000                  
2013 Employee Stock Purchase Plan [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Common stock remain available for future grant 200,000                       200,000          
Number of shares approved under the plan         300,000         300,000                
Term of award                         24 months          
Percentage of closing price of common stock                         85.00%          
2013 Employee Stock Purchase Plan [Member] | Minimum [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Share-based compensation award, expiration date                         Feb. 28, 2018          
2013 Employee Stock Purchase Plan [Member] | Maximum [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Share-based compensation award, expiration date                         Aug. 31, 2019          
2014 Employment Commencement Incentive Plan (2014 Plan) [Member]                                    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                                    
Common stock remain available for future grant 600,000                       600,000          
Number of shares approved under the plan                                 600,000  
Number of additional shares approved under the plan   3,800,000         1,000,000                      
[1] In June 2017, the Company granted its new CEO 3,300,000 options with service and market conditions. These options have a five-year cliff vesting schedule. The fair value of $13.48 for these options was determined by a lattice model with Monte Carlo simulations. The remaining are service-based options which have a four-year vesting schedule with 25% vesting on the first anniversary and 1/48 monthly thereafter.