Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Operating expenses:
Research and development	$ 8,295	$ 2,437	$ 18,329	$ 16,981
General and administrative	2,738	1,043	6,053	2,327
Total operating expenses	11,033	3,480	24,382	19,308
Loss from operations	(11,033)	(3,480)	(24,382)	(19,308)
Other income (expense), net:
Change in fair value of preferred stock warrants	(7,487)	(503)	(17,443)	(999)
Other income (expense)	29	22	48	55
Total other income (expense), net	(7,458)	(481)	(17,395)	(944)
Loss before provision for income taxes	(18,491)	(3,961)	(41,777)	(20,252)
Provision for income tax	14		25
Net loss	(18,505)	(3,961)	(41,802)	(20,252)
Adjustment to redemption value on redeemable convertible preferred stock	(222,130)	(5,591)	(374,015)	(44,866)
Net loss attributable to common stockholders	$ (266,544)	$ (9,552)	$ (441,726)	$ (65,118)
Net loss per share attributable to common stockholders, basic and diluted (Note 3)	$ (11.03)	$ (9.37)	$ (48.36)	$ (69.95)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (Note 3)	24,167,238	1,019,553	9,133,978	930,922
Series B and B-1 [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend	$ (5,543)		$ (5,543)
Series C and D [Member]
Other income (expense), net:
Redeemable convertible preferred stock dividend	$ (20,366)		$ (20,366)