Selected Quarterly Financial Data - Quarterly Financial Data (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
Selected Quarterly Financial Data [Line Items]
Total revenue	$ 33,862	$ 29,218	$ 23,315	$ 28,222	$ 39,826	$ 27,762	$ 1,087,717	$ 38,018	$ 114,617	$ 1,193,323	$ 307,711
Cost of goods sold	5,989	4,927	5,018	5,440	7,461	4,783	5,522	6,646	21,374	24,412	30,547
Research and development expenses	110,369	99,048	106,686	118,463	108,883	102,895	88,334	99,424	434,566	399,536	268,461
Operating income (loss)	(109,638)	(98,740)	(110,970)	(120,687)	(100,295)	(98,634)	973,600	(86,739)	(440,035)	687,932	(59,642)
Net income (loss) (1)	$ (112,164)	$ (98,585)	$ (110,286)	$ (119,632)	$ (98,212)	$ (96,143)	$ 971,460	$ (95,792)	$ (440,667)	$ 681,313	$ (96,692)
Net income (loss) per share
Basic ( in dollars per share)	$ (0.64)	$ (0.56)	$ (0.63)	$ (0.69)	$ (0.57)	$ (0.56)	$ 5.67	$ (0.60)	$ (2.52)	$ 4.02	$ (0.62)
Diluted (in dollars per share)	$ (0.64)	$ (0.56)	$ (0.63)	$ (0.69)	$ (0.57)	$ (0.56)	$ 5.33	$ (0.60)	$ (2.52)	$ 3.78	$ (0.62)
Product Sales
Selected Quarterly Financial Data [Line Items]
Total revenue	$ 5,815	$ 5,558	$ 4,346	$ 4,398	$ 4,360	$ 4,256	$ 5,863	$ 6,295	$ 20,117	$ 20,774	$ 32,688