Financial Information by Quarter (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Product, net	$ 84,415	$ 78,486	$ 73,529	$ 64,604	$ 57,277	$ 45,954	$ 35,011	$ 16,342	$ 301,034	$ 154,584	$ 5,421
Type of Revenue [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Total revenues	$ 84,415	$ 78,486	$ 73,529	$ 64,604	$ 57,277	$ 45,954	$ 35,011	$ 16,342	$ 301,034	$ 154,584	$ 5,421
Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)	13,135	8,741	6,735	5,582	3,546	3,078	505	224	34,193	7,353	101
Research and development	146,207	86,584	122,848	46,204	44,441	34,239	58,908	29,119	401,843	166,707	188,272
Selling, general and administrative	64,220	53,044	47,156	43,341	32,221	28,176	36,069	26,216	207,761	122,682	83,749
EXONDYS 51 settlement and license charges						25,588	2,839			28,400
Amortization of in-licensed rights	216	216	217	216	216	780	29	28	865	1,053	29
Total cost and expenses	223,778	148,585	176,956	95,343	80,424	91,861	98,350	55,587	644,662	326,222	272,151
Operating loss	(139,363)	(70,099)	(103,427)	(30,739)	(23,147)	(45,907)	(63,339)	(39,245)	(343,628)	(171,638)	(266,730)
Other (loss) income:
Interest (expense) income and other, net	(2,311)	(6,968)	(5,218)	(4,485)	(2,693)	184	184	335	(18,982)	(1,990)	(535)
Gain from sale of Priority Review Voucher								125,000		125,000
Total other (loss) income	(2,311)	(6,968)	(5,218)	(4,485)	(2,693)	184	184	125,335	(18,982)	123,010	(535)
Loss before income tax (benefit) expense	(141,674)	(77,067)	(108,645)	(35,224)	(25,840)	(45,723)	(63,155)	86,090	(362,610)	(48,628)	(267,265)
Income tax (benefit) expense	(779)	(674)	622	139	(1,842)	2,011	(109)	2,000	(692)	2,060
Net loss	$ (140,895)	$ (76,393)	$ (109,267)	$ (35,363)	$ (23,998)	$ (47,734)	$ (63,046)	$ 84,090	$ (361,918)	$ (50,688)	$ (267,265)
Net loss per share — basic and diluted	$ (2.05)	$ (1.15)	$ (1.67)	$ (0.55)					$ (5.46)	$ (0.86)	$ (5.49)
Weighted average number of shares of common stock used in computing basic and diluted net loss per share	68,653	66,209	65,484	64,631					66,250	58,818	48,697
Net income (loss) per share:
Basic (loss) earnings per share					$ (0.37)	$ (0.78)	$ (1.15)	$ 1.53
Diluted (loss) earnings per share					$ (0.37)	$ (0.78)	$ (1.15)	$ 1.50
Weighted average number of shares of common stock used in calculating:
Weighted-average number of shares of common stock outstanding for computing basic loss per share					64,277	61,528	54,976	54,850	66,250	58,818	48,697
Diluted (loss) earnings per share					64,277	61,258	54,976	56,012