Consolidated Statements of Operations and Comprehensive Loss - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues:
Product, net	$ 301,034	$ 154,584	$ 5,421
Type of Revenue [Extensible List]	us-gaap:ProductMember	us-gaap:ProductMember	us-gaap:ProductMember
Total revenues	$ 301,034	$ 154,584	$ 5,421
Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)	34,193	7,353	101
Research and development	401,843	166,707	188,272
Selling, general and administrative	207,761	122,682	83,749
EXONDYS 51 litigation and license charges		28,427
Amortization of in-licensed rights	865	1,053	29
Total cost and expenses	644,662	326,222	272,151
Operating loss	(343,628)	(171,638)	(266,730)
Other (loss) income:
Interest expense and other, net	(18,982)	(1,990)	(535)
Gain from sale of Priority Review Voucher		125,000
Total other (loss) income	(18,982)	123,010	(535)
Loss before income tax (benefit) expense	(362,610)	(48,628)	(267,265)
Income tax (benefit) expense	(692)	2,060
Net loss	(361,918)	(50,688)	(267,265)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	280	(259)	(9)
Total other comprehensive gain (loss)	280	(259)	(9)
Comprehensive loss	$ (361,638)	$ (50,947)	$ (267,274)
Net loss per share — basic and diluted	$ (5.46)	$ (0.86)	$ (5.49)
Weighted average number of shares of common stock outstanding for computing basic and diluted net loss per share	66,250	58,818	48,697