Selected Quarterly Financial Information (unaudited) (Details) - 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
Quarterly Financial Information Disclosure [Abstract]
Collaboration revenue	$ 0	$ 0	$ 0	$ 0	$ 0	$ 206,654	$ 1,661	$ 1,662	$ 0	$ 209,977	$ 50,909
Research and development expenses	16,128	9,407	8,516	7,686	7,946	10,707	15,657	31,979	41,737	66,289	196,295
General and administrative expenses	5,667	5,968	6,332	5,645	6,913	7,059	8,552	13,159	23,612	35,683	50,178
Income (loss) from operations	(21,795)	(15,375)	(14,848)	(13,331)	(14,859)	188,888	(22,548)	(43,476)	$ (65,349)	$ 108,005	$ (195,564)
Net income (loss) attributable to common stockholders	$ 104,114	$ (14,745)	$ (13,209)	$ (13,073)	$ (9,542)	$ 189,073	$ (22,204)	$ (43,122)
Basic (in dollars per share)	$ 2.62	$ (0.41)	$ (0.37)	$ (0.36)	$ (0.26)	$ 5.26	$ (0.62)	$ (1.20)	$ 1.70	$ 3.18	$ (5.45)
Dilutive (in dollars per share)	$ 2.62	$ (0.41)	$ (0.37)	$ (0.36)	$ (0.26)	$ 5.25	$ (0.62)	$ (1.20)	$ 1.70	$ 3.17	$ (5.45)