Statement of Operations and Comprehensive Loss - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Collaboration revenue		$ 17,334
Operating Expenses [Abstract]
Research and development	$ 2,316	27,679	$ 8,898
General and administrative	1,450	9,909	5,469
Total operating expenses	3,766	37,588	14,367
Operating loss	(3,766)	(20,254)	(14,367)
Other expense, net
Interest income (expense), net	(67)	332	(1)
Other financing expense		(9,750)	(1,949)
Total other expense, net	(67)	(9,418)	(1,950)
Net loss	(3,833)	(29,672)	(16,317)
Other comprehensive loss
Net unrealized loss on available for-sale securities		(251)
Total other comprehensive loss		(251)
Comprehensive loss	(3,833)	(29,923)	(16,317)
Reconciliation of net loss to net loss attributalbe to common stockholder:
Accretion of redeemable convertible preferred stock to redemption value		(7,373)	(1,366)
Accrued dividends on series A preferred stock		(1,245)
Net loss attributable to common stockholders	$ (3,833)	$ (38,290)	$ (17,683)
Net loss per share attributable to common stock, basic and diluted	$ (1,629.68)	$ (9.14)	$ (27.83)
Weighted-average common shares outstanding, basic and diluted	2,352	4,191,210	635,448