Selected quarterly financial data (unaudited) - 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 revenues	$ 10,708	$ 19,273	$ 198,890	$ 21,863	$ 9,997	$ 8,910	$ 13,296	$ 12,471	$ 250,734	$ 44,674	$ 54,579
Total operating expenses	214,690	208,967	224,835	225,288	240,531	219,326	215,998	183,645	873,780	859,500	626,602
Loss from operations	(203,982)	(189,694)	(25,945)	(203,425)	(230,534)	(210,416)	(202,702)	(171,174)	(623,046)	(814,826)	(572,023)
Net loss	$ (199,874)	$ (194,745)	$ (21,465)	$ (202,611)	$ (223,347)	$ (206,033)	$ (195,782)	$ (164,446)	$ (618,695)	$ (789,608)	$ (555,625)
Net loss per share applicable to common stockholders - basic and diluted (in dollars per share)	$ (3.01)	$ (2.94)	$ (0.36)	$ (3.64)	$ (4.04)	$ (3.73)	$ (3.55)	$ (2.99)	$ (9.95)	$ (14.31)	$ (10.68)