Assumptions for Measuring Fair Values of Stocks (Detail)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected dividend yield		0.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Risk-free interest rate, minimum	[1]	1.60%	1.10%	1.10%
Risk-free interest rate, maximum	[1]	2.10%	1.80%	1.70%
Expected volatility, minimum	[2]	54.00%	78.20%	94.30%
Expected volatility, maximum	[2]	63.00%	137.10%	111.10%
Stock Options [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected terms	[3]	4 years 2 months 12 days	4 years 2 months 12 days	4 years 8 months 12 days
Stock Options [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected terms	[3]	4 years 9 months 18 days	4 years 9 months 18 days	5 years

[1] The risk-free interest rate is estimated using an average of Treasury bill interest rates over a historical period commensurate with the expected term of the option that correlates to the prevailing interest rates at the time of grant.

[2] Upon commercialization of EXONDYS 51 in the U.S., the Company’s future risk profile changed. As a result, starting January 1, 2017, the Company estimates expected volatility by using only implied volatility in exchange-traded options of the Company’s common stock. Prior to January 1, 2017, the expected volatility was estimated using a blend of calculated volatility of the Company’s common stock over a historical period and implied volatility in exchange-traded options of the Company’s common stock.

[3] The expected terms are estimated using historical exercise behavior.