Selected Quarterly Financial Information (unaudited) (Details) - 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
Quarterly Financial Information Disclosure [Abstract]
Research and development expenses	$ 11,567	$ 10,383	$ 10,009	$ 7,685	$ 16,128	$ 9,407	$ 8,516	$ 7,686	$ 39,644	$ 41,737	$ 66,289
General and administrative expenses	6,275	4,674	5,198	5,481	5,667	5,968	6,332	5,645	21,628	23,612	35,683
Income (loss) from operations	(17,842)	(15,057)	(15,207)	(13,166)	(21,795)	(15,375)	(14,848)	(13,331)	$ (61,272)	$ (65,349)	$ 108,005
Net income (loss) attributable to common stockholders	$ (17,478)	$ (14,437)	$ (14,443)	$ (12,501)	$ 104,114	$ (14,745)	$ (13,209)	$ (13,073)
Basic (in dollars per share)	$ (0.39)	$ (0.35)	$ (0.35)	$ (0.30)	$ 2.62	$ (0.41)	$ (0.37)	$ (0.36)	$ (1.39)	$ 1.70	$ 3.18
Dilutive (in dollars per share)	$ (0.39)	$ (0.35)	$ (0.35)	$ (0.30)	$ 2.62	$ (0.41)	$ (0.37)	$ (0.36)	$ (1.39)	$ 1.70	$ 3.17