Assumptions for Measuring Fair Values of Stocks (Detail)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
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]	2.50%	1.60%	1.10%
Risk-free interest rate, maximum	[1]	3.00%	2.10%	1.80%
Expected term	[2]	5 years 21 days
Expected volatility, minimum	[3]	52.40%	54.00%	78.20%
Expected volatility, maximum	[3]	60.08%	63.00%	137.10%
Stock Options [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	[2]		4 years 2 months 12 days	4 years 2 months 12 days
Stock Options [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expected term	[2]		4 years 9 months 18 days	4 years 9 months 18 days

[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] The expected term is estimated using historical exercise behavior.

[3] 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.