Condensed Consolidated Statements of Operations - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Operating expenses:
Research and development	$ 4,738	$ 12,717	$ 10,034	$ 14,544
General and administrative	1,874	791	3,315	1,284
Total operating expenses	6,612	13,508	13,349	15,828
Loss from operations	(6,612)	(13,508)	(13,349)	(15,828)
Other income (expense), net:
Change in fair value of preferred stock warrants	(8,630)	(347)	(9,956)	(496)
Other income (expense)	(6)	31	19	33
Total other income (expense), net	(8,636)	(316)	(9,937)	(463)
Loss before provision for income taxes	(15,248)	(13,824)	(23,286)	(16,291)
Provision for income tax	1		11
Net loss	(15,249)	(13,824)	(23,297)	(16,291)
Adjustment to redemption value on redeemable convertible preferred stock	(140,880)	(36,833)	(151,885)	(39,275)
Net loss attributable to common stockholders	$ (156,129)	$ (50,657)	$ (175,182)	$ (55,566)
Net loss per share attributable to common stockholders, basic and diluted (Note 3)	$ (104.10)	$ (55.04)	$ (117.35)	$ (62.72)
Weighted-average shares used in computing net loss per share attributable to common stockholders, basic and diluted (Note 3)	1,499,840	920,441	1,492,764	885,872