Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (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
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 370	$ 148	$ 44	$ 157	$ 77,016	$ 211,928	$ 318	$ 328	$ 719	$ 289,590	$ 3,124
Total operating expenses	108,131	92,547	80,733	73,743	66,033	72,765	55,301	48,751	355,154	242,850	162,067
Loss from operations	(107,761)	(92,399)	(80,689)	(73,586)	10,983	139,163	(54,983)	(48,423)	(354,435)	46,740	(158,943)
Net (loss) income	$ (107,039)	$ (92,439)	$ (79,656)	$ (69,731)	$ 30,542	$ 138,423	$ (53,699)	$ (48,408)	$ (348,865)	$ 66,858	$ (164,981)
Net (loss) income per common share:
Basic	$ (1.50)	$ (1.32)	$ (1.30)	$ (1.15)	$ 0.53	$ 2.52	$ (1.01)	$ (0.93)	$ (5.29)	$ 1.23	$ (3.44)
Diluted	$ (1.50)	$ (1.32)	$ (1.30)	$ (1.15)	$ 0.51	$ 2.40	$ (1.01)	$ (0.93)	$ (5.29)	$ 1.17	$ (3.44)
Weighted-average common shares outstanding:
Basic	71,282,096	70,143,481	61,420,746	60,847,683	57,395,839	54,829,057	53,188,041	52,093,208	65,949,672	54,392,304	47,964,368
Diluted	71,282,096	70,143,481	61,420,746	60,847,683	60,233,927	57,598,901	53,188,041	52,093,208	65,949,672	56,932,798	47,964,368