Condensed Consolidated Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Operating expenses:
Research and development	$ 55,900	$ 26,096	$ 101,100	$ 49,677
General and administrative	14,954	8,910	27,234	16,044
Total operating expenses	70,854	35,006	128,334	65,721
Loss from operations	(70,854)	(35,006)	(128,334)	(65,721)
Interest income, net	672	266	1,379	442
Other expense, net	(20)	(7)	(24)	(11)
Net loss	$ (70,202)	$ (34,747)	$ (126,979)	$ (65,290)
Net loss per share—basic and diluted	$ (1.88)	$ (1.08)	$ (3.40)	$ (2.05)
Weighted average common shares outstanding—basic and diluted	37,361,129	32,062,298	37,315,393	31,835,194
Comprehensive loss:
Net loss	$ (70,202)	$ (34,747)	$ (126,979)	$ (65,290)
Other comprehensive items:
Unrealized gain on marketable securities	14	39	35	39
Total other comprehensive gain	14	39	35	39
Total comprehensive loss	$ (70,188)	$ (34,708)	$ (126,944)	$ (65,251)