Statement of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Collaboration revenue	$ 14,220,000	$ 17,334,000
Operating Expenses [Abstract]
Research and development	42,249,000	27,679,000	$ 8,898,000
General and administrative	13,270,000	9,909,000	5,469,000
Total operating expenses	55,519,000	37,588,000	14,367,000
Operating loss	(41,299,000)	(20,254,000)	(14,367,000)
Other expense, net
Interest income	976,000	332,000	(1,000)
Other expense, net	182,000	(9,750,000)	(1,949,000)
Total other expense, net	1,158,000	(9,418,000)	(1,950,000)
Loss before income taxes	(40,141,000)	(29,672,000)	(16,317,000)
Income tax benefit	52,000	0
Net loss	(40,193,000)	(29,672,000)	(16,317,000)
Other comprehensive loss
Net unrealized gain (loss) on available-for-sale-securities, net	199,000	(251,000)
Other comprehensive loss	199,000	(251,000)
Comprehensive loss	(39,994,000)	(29,923,000)	(16,317,000)
Reconciliation of net loss to net loss attributable to common stockholder:
Accretion of redeemable convertible preferred stock to redemption value		(7,373,000)	(1,366,000)
Accrued dividends on series A preferred stock		(1,245,000)
Net loss attributable to common stockholders	$ 40,193,000	$ 38,290,000	$ 17,683,000
Net loss per share attributable to common stock, basic and diluted	$ 1.59	$ 9.14	$ 27.83
Weighted-average common shares outstanding, basic and diluted	25,302,414	4,191,210	635,448