Financial Information by Quarter (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
Revenues:
Revenue	$ 122,644	$ 121,429	$ 111,344	$ 100,448	$ 100,113	$ 99,041	$ 94,668	$ 87,011	$ 455,865		$ 380,833	$ 301,034
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
Collaboration	$ 22,494	$ 22,495	$ 26,019	$ 13,226					84,234
Total revenues	145,138	143,924	137,363	113,674	$ 100,113	$ 99,041	$ 94,668	$ 87,011	540,099		$ 380,833	$ 301,034
Cost and expenses:
Cost of sales (excluding amortization of in-licensed rights)	22,404	15,015	13,341	12,622	15,567	13,037	15,919	12,063	63,382		56,586	34,193
Research and development expense	207,239	190,438	188,522	136,144	223,141	133,949	113,266	90,553
Selling, general and administrative	86,046	75,373	73,688	82,768	81,424	75,429	67,393	60,566	317,875		284,812	207,761
Acquired in-process research and development							173,240				173,240
Settlement and license charges					10,000						10,000
Amortization of in-licensed rights	165	166	165	166	200	216	217	216	662		849	865
Total cost and expenses	315,854	280,992	275,716	231,700	330,332	222,631	370,035	163,398	1,104,262		1,086,396	644,662
Operating loss	(170,716)	(137,068)	(138,353)	(118,026)	(230,219)	(123,590)	(275,367)	(76,387)	(564,163)		(705,563)	(343,628)
Other (loss) income:
Gain from sale of Priority Review Voucher				108,069					108,069
Loss on contingent consideration		(45,000)							(45,000)	[1]
Other expense, net	(17,769)	(14,335)	(12,447)	(7,420)	(4,773)	(2,510)	(862)	(172)
Total other income (loss)	(17,769)	(59,335)	(12,447)	100,649	(4,773)	(2,510)	(862)	(172)	11,098		(8,317)	(18,982)
Loss before income tax expense (benefit)	(188,485)	(196,403)	(150,800)	(17,377)	(234,992)	(126,100)	(276,229)	(76,559)	(553,065)		(713,880)	(362,610)
Income tax expense (benefit)	832	96	20	115	711	226	174	84	1,063		1,195	(692)
Net loss	$ (189,317)	$ (196,499)	$ (150,820)	$ (17,492)	$ (235,703)	$ (126,326)	$ (276,403)	$ (76,643)	$ (554,128)		$ (715,075)	$ (361,918)
Net loss per share — basic and diluted	$ (2.40)	$ (2.50)	$ (1.93)	$ (0.23)	$ (3.16)	$ (1.70)	$ (3.74)	$ (1.07)	$ (7.11)		$ (9.71)	$ (5.46)
Weighted average number of shares of common stock used in computing basic and diluted net loss per share	78,905	78,501	77,968	76,432	74,557	74,177	73,958	71,731	77,956,000		73,615,000	66,250,000

[1] The loss on contingent consideration is related to the fair value adjustment of the regulatory-related contingent payments that are accounted for as derivatives. Please see Note 5. Fair Value Measurements for further details.