Financial Information by Quarter (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Product, net	$ 100,113	$ 99,041	$ 94,668	$ 87,011	$ 84,415	$ 78,486	$ 73,529	$ 64,604	$ 380,833	$ 301,034	$ 154,584
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	$ 100,113	$ 99,041	$ 94,668	$ 87,011	$ 84,415	$ 78,486	$ 73,529	$ 64,604	$ 380,833	$ 301,034	$ 154,584
Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)	15,567	13,037	15,919	12,063	13,135	8,741	6,735	5,582	56,586	34,193	7,353
Research and development expense	223,141	133,949	113,266	90,553	146,207	86,584	122,848	46,204
Selling, general and administrative	81,424	75,429	67,393	60,566	64,220	53,044	47,156	43,341	284,812	207,761	122,682
Acquired in-process research and development			173,240						173,240
Settlement and license charges	10,000								10,000		28,427
Amortization of in-licensed rights	200	216	217	216	216	216	217	216	849	865	1,053
Total cost and expenses	330,332	222,631	370,035	163,398	223,778	148,585	176,956	95,343	1,086,396	644,662	326,222
Operating loss	(230,219)	(123,590)	(275,367)	(76,387)	(139,363)	(70,099)	(103,427)	(30,739)	(705,563)	(343,628)	(171,638)
Other (loss) income:
Other expense, net	(4,773)	(2,510)	(862)	(172)
Total other (loss) income	(4,773)	(2,510)	(862)	(172)	(2,311)	(6,968)	(5,218)	(4,485)	(8,317)	(18,982)	123,010
Interest expense and other, net					(2,311)	(6,968)	(5,218)	(4,485)
Loss before income tax expense (benefit)	(234,992)	(126,100)	(276,229)	(76,559)	(141,674)	(77,067)	(108,645)	(35,224)	(713,880)	(362,610)	(48,628)
Income tax expense (benefit)	711	226	174	84	(779)	(674)	622	139	1,195	(692)	2,060
Gain from sale of Priority Review Voucher											125,000
Net loss	$ (235,703)	$ (126,326)	$ (276,403)	$ (76,643)	$ (140,895)	$ (76,393)	$ (109,267)	$ (35,363)	$ (715,075)	$ (361,918)	$ (50,688)
Net loss per share — basic and diluted	$ (3.16)	$ (1.70)	$ (3.74)	$ (1.07)	$ (2.05)	$ (1.15)	$ (1.67)	$ (0.55)	$ (9.71)	$ (5.46)	$ (0.86)
Weighted average number of shares of common stock used in computing basic and diluted net loss per share	74,557	74,177	73,958	71,731	68,653	66,209	65,484	64,631	73,615,000	66,250,000	58,818,000