CONDENSED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS
Alliance revenue	$ 902	$ 16,199	$ 249
Operating expenses:
Research and development	50,653	39,824	33,699
General and administrative	16,716	15,132	15,396
Total operating expenses	67,369	54,956	49,095
Loss from operations	(66,467)	(38,757)	(48,846)
Other income (expense):
Interest income	574	415	357
Interest expense	(50)	(80)	(105)
Foreign currency exchange (loss) gain	(41)	33	39
Net loss	(65,984)	(38,389)	(48,555)
Net loss	$ (65,984)	$ (38,389)	$ (48,555)
Net loss per share applicable to common stockholders - basic and diluted (Note 15)	$ (0.42)	$ (0.30)	$ (0.42)
Weighted-average number of common shares used in computing net loss per share applicable to common stockholders - basic and diluted	157,398	127,597	115,092
Net loss	$ (65,984)	$ (38,389)	$ (48,555)
Other comprehensive loss:
Unrealized gain (loss) on available-for-sale securities	17	117	(117)
Total other comprehensive income (loss)	17	117	(117)
Comprehensive loss	$ (65,967)	$ (38,272)	$ (48,672)