Selected Quarterly Financial Data - Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Data [Line Items]
Total revenue	$ 39,826	$ 27,762	$ 1,087,717	$ 38,018	$ 95,466	$ 152,928	$ 34,589	$ 24,728	$ 1,193,323	$ 307,711	$ 165,436
Cost of goods sold	7,461	4,783	5,522	6,646	9,753	5,674	8,989	6,131	24,412	30,547	30,215
Research and development expenses	108,883	102,895	88,334	99,424	81,429	65,714	60,260	61,058	399,536	268,461	203,801
Operating income (loss)	(100,295)	(98,634)	973,600	(86,739)	(24,034)	69,485	(50,656)	(54,437)	687,932	(59,642)	(112,855)
Net income (loss)	$ (98,212)	$ (96,143)	$ 971,460	$ (95,792)	$ (33,826)	$ 60,871	$ (59,871)	$ (63,866)	$ 681,313	$ (96,692)	$ (153,524)
Net income (loss) per share
Basic	$ (0.57)	$ (0.56)	$ 5.67	$ (0.60)	$ (0.21)	$ 0.39	$ (0.39)	$ (0.42)	$ 4.02	$ (0.62)	$ (1.10)
Diluted	$ (0.57)	$ (0.56)	$ 5.33	$ (0.60)	$ (0.21)	$ 0.37	$ (0.39)	$ (0.42)	$ 3.78	$ (0.62)	$ (1.10)
Product Sales [Member]
Selected Quarterly Financial Data [Line Items]
Total revenue	$ 4,360	$ 4,256	$ 5,863	$ 6,295	$ 7,791	$ 4,448	$ 15,693	$ 4,756	$ 20,774	$ 32,688	$ 55,354