STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement Of Income And Comprehensive Income [Abstract]
Revenues:	$ 7,712	$ 5,077	$ 888
Operating expenses:
Research and development	28,357	28,302	10,890
General and administrative	12,558	6,540	4,954
Total operating expenses	40,915	34,842	15,844
Loss from operations	(33,203)	(29,765)	(14,956)
Interest income	1,315	7	6
Interest expense	(1,732)	(487)	(254)
Other income (expense), net	(1,744)	(55)	71
Loss before provision for income taxes	(35,364)	(30,300)	(15,133)
Provision for income taxes	10	10	10
Net loss	(35,374)	(30,310)	(15,143)
Accretion to redemption value and cumulative dividends on preferred stock	(6,705)	(4,566)	(3,751)
Net loss attributable to common stockholders	$ (42,079)	$ (34,876)	$ (18,894)
Net loss per share attributable to common stockholders, basic and diluted	$ (4.90)	$ (35.25)	$ (24.46)
Shares used to compute net loss per share attributable to common stockholders, basic and diluted	8,595,247	989,453	772,320
Other comprehensive loss:
Changes in unrealized losses on short-term investments	$ (76)
Total other comprehensive loss	(76)
Comprehensive loss	$ (35,450)	$ (30,310)	$ (15,143)