Summary of Stock Awards Granted (Detail) - $ / shares	3 Months Ended			9 Months Ended
	Sep. 30, 2016		Sep. 30, 2015	Sep. 30, 2016		Sep. 30, 2015
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Stock options	1,050		702,067	1,214,426	[1]	2,676,778
Weighted Average Grant Date Fair Value, Stock options	$ 37.38		$ 24.05	$ 11.96		$ 14.67
Restricted Stock Awards (RSAs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Grants, Restricted stock awards	91,778	[2]	65,000	117,553	[3]	181,783
Weighted Average Grant Date Fair Value, Restricted stock awards	$ 48.94		$ 33.81	$ 41.22		$ 20.80

[1] Included in 2016 stock option grants are 287,500 options with performance conditions. As a result of the approval of EXONDYS 51, 25% of these performance grants vested immediately and another 25% were triggered to be eligible for vesting subject to the remaining service conditions of the awards. As of September 30, 2016, the performance conditions of the remaining 50% were not probable of being achieved. The remaining stock options granted during the periods presented in the table have only service-based criteria and vest over four years.

[2] The Company granted certain employees 91,778 restricted stock awards (“RSA”) with certain sales targets. If and when deemed probable that such performance milestones may be achieved within the required time frame, the Company may recognize up to $4.5 million of stock-based compensation related to these grants.

[3] Included in the 2016 RSA grants are 18,755 shares granted to certain employees in lieu of a portion of their 2015 annual bonus payments. These RSA grants were fully vested as of September 30, 2016. The remaining RSAs will be fully vested by June 2017.