Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Operating activities
Net loss	$ (55,748)	$ (56,680)	$ (18,668)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	1,591	1,490	1,360
Loss from valuation of convertible note payable	1,811	12,118	1,145
Stock-based compensation	15,368	7,039	3,422
Amortization of premium on investments, net	1,472	1,597	1,439
Loss on disposal of long-term assets	98	18	0
Change in operating assets and liabilities:
Contracts and other receivables	(9,746)	(196)	(79)
Prepaid expenses	(3,967)	(1,795)	(1,890)
Other assets	(176)	(87)	(393)
Accounts payable	529	940	860
Accrued liabilities	2,094	556	1,355
Accrued compensation	284	811	(51)
Deferred revenue	(3,090)	(5,039)	(16,819)
Deferred rent and other liabilities	(379)	(282)	(11)
Net cash used in operating activities	(49,859)	(39,510)	(28,330)
Investing activities
Purchases of short-term investments	(78,401)	(113,417)	(72,005)
Sales and maturities of short-term investments	101,306	85,421	31,951
Purchases of property and equipment	(1,363)	(1,146)	(800)
Acquisition of intangibles	(67)	(65)	(35)
Net cash provided by (used in) investing activities	21,475	(29,207)	(40,889)
Financing activities
Proceeds from issuance of common stock, net	492	86,146	45,980
Proceeds from exercise of common stock options	6,680	2,233	594
Principal payments on other long-term obligations	(155)	(142)	(100)
Net cash provided by financing activities	7,017	88,237	46,474
Net (decrease) increase in cash and cash equivalents	(21,367)	19,520	(22,745)
Cash and cash equivalents at beginning of period	37,327	17,807	40,552
Cash and cash equivalents at end of period	15,960	37,327	17,807
Supplemental disclosure of cash flow information
Net changes in restricted cash	1,256	0	0
Interest paid	(27)	(39)	(37)
Income taxes paid	(1)	(1)	(1)
Supplemental disclosure of non-cash investing and financing activities
Allowance for tenant improvements	1,656	0	653
Amounts accrued for property and equipment	223	76	0
Amounts accrued for patent expenditures	$ 28	$ 42	$ 11