Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenues:
Total revenue	$ 5,281	$ 2,607	$ 42,000
Cost of revenue	600	466	8,400
Gross profit	4,681	2,141	33,600
Operating expenses:
Research and development	71,677	69,373	75,484
General and administrative	24,267	23,715	23,231
Total operating expenses	95,944	93,088	98,715
Loss from operations	(91,263)	(90,947)	(65,115)
Interest expense	(5,726)	(3,534)
Other income, net	2,352	3,187	1,955
Loss before provision for income taxes	(94,637)	(91,294)	(63,160)
Provision for income taxes	303	4	1,179
Net loss	$ (94,940)	$ (91,298)	$ (64,339)
Net loss per common share, basic and diluted	$ (1.47)	$ (1.62)	$ (1.36)
Shares used in computing net loss per share - basic and diluted	64,478,066	56,219,919	47,435,331
Comprehensive loss:
Net loss	$ (94,940)	$ (91,298)	$ (64,339)
Unrealized gains on available-for-sale securities	58	9	24
Comprehensive loss	(94,882)	(91,289)	(64,315)
Licensing
Revenues:
Total revenue	4,500	2,320	$ 42,000
Collaborative development
Revenues:
Total revenue	459
Other
Revenues:
Total revenue	$ 322	$ 287