Consolidated 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]
Revenue	$ 0
Operating expenses:
Research and development expenses	38,095	$ 10,718	$ 4,805
General and administrative expenses	16,761	4,364	1,247
Total operating expenses	54,856	15,082	6,052
Loss from operations	(54,856)	(15,082)	(6,052)
Other income (expense):
Interest income	638
Interest expense	(555)	(209)	(42)
Revaluation of preferred stock warrant liability	(7)	(1,418)	(8)
Total other income (expense), net	76	(1,627)	(50)
Net loss	(54,780)	(16,709)	(6,102)
Accretion of convertible preferred stock to redemption value		(1,291)	(875)
Net loss attributable to common stockholders	$ (54,780)	$ (18,000)	$ (6,977)
Net loss per share attributable to common stockholders, basic and diluted	$ (2.33)	$ (2.67)	$ (1.09)
Weighted average common shares outstanding, basic and diluted	23,532,400	6,748,037	6,394,916
Other comprehensive income (loss):
Unrealized gain on investments, net of tax of $0	$ 30
Total other comprehensive income	30
Comprehensive loss	$ (54,750)	$ (18,000)	$ (6,977)