Consolidated Statements of Operations - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Collaboration revenue	$ 26,117	$ 16,479	$ 6,044
Operating expenses:
Research and development	67,647	31,840	11,170
General and administrative	28,025	16,798	8,283
Total operating expenses	95,672	48,638	19,453
Operating loss	(69,555)	(32,159)	(13,409)
Interest income	2,012	525
Loss before income taxes	(67,543)	(31,634)	(13,409)
Income tax benefit			1,012
Net loss	$ (67,543)	$ (31,634)	$ (12,397)
Net loss per share, basic and diluted	$ (1.88)	$ (1.42)	$ (51.02)
Weighted average shares outstanding, basic and diluted	36,006	22,222	243