Selected Quarterly Financial Information (Unaudited) (Details) - 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
Selected Quarterly Financial Information (Unaudited) [Abstract]
Total Revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Total operating expenses	59,237	27,387	23,559	24,120	19,649	19,165	18,392	18,945	134,303	76,151	71,156
Loss from operations	(59,237)	(27,387)	(23,559)	(24,120)	(19,649)	(19,165)	(18,392)	(18,945)	(134,303)	(76,151)	(71,156)
Net loss	$ (60,916)	$ (29,080)	$ (25,040)	$ (24,664)	$ (19,855)	$ (19,284)	$ (18,680)	$ (19,451)	$ (139,700)	$ (77,270)	$ (74,518)
Net loss per share attributable to common stockholders - basic and diluted (in dollars per share)	$ (1.09)	$ (0.53)	$ (0.45)	$ (0.45)	$ (0.39)	$ (0.38)	$ (0.38)	$ (0.43)	$ (2.52)	$ (1.58)	$ (1.89)