Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Operating activities
Net loss	$ (18,668)	$ (17,408)	$ (7,602)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization expense	1,360	1,017	911
Loss from change in value of convertible note payable	1,145	2,969
Loss on extinguishment of debt		1,738
Amortization of premium on investments, net	1,464	422	551
Gain on investments	(25)	(28)	(1)
Stock-based compensation	3,422	1,550	825
Loss on disposal of long-term assets		36
Change in operating assets and liabilities:
Contracts and other receivables	(79)
Prepaid and other assets	(2,283)	(308)	(136)
Accounts payable	860	(189)	(1,345)
Accrued compensation	(51)	677	(528)
Accrued liabilities	1,355	(20)	29
Deferred revenue	(16,819)	486	(8,219)
Deferred rent and other liabilities	(11)	337	452
Net cash used in operating activities	(28,330)	(8,721)	(15,063)
Investing activities
Purchases of short-term investments	(72,005)	(62,041)	(50,663)
Maturities and sales of short-term investments	31,951	33,083	54,585
Purchases of property and equipment	(800)	(1,151)	(467)
Acquisition of intangibles	(35)	(275)	(131)
Net cash (used in) provided by investing activities	(40,889)	(30,384)	3,324
Financing activities
Proceeds from issuance of convertible notes payable and other long-term obligations		5,000
Proceeds from issuance of common stock, net	45,980	65,788
Principal payments on other long-term obligations	(100)	(445)	(412)
Proceeds from exercise of common stock options	594	139	58
Net cash provided by (used in) financing activities	46,474	70,482	(354)
Net (decrease) increase in cash and cash equivalents	(22,745)	31,377	(12,093)
Cash and cash equivalents at beginning of period	40,552	9,175	21,268
Cash and cash equivalents at end of period	17,807	40,552	9,175
Supplemental disclosure of cash flow information
Interest paid	37	8	65
Income taxes paid	1	208
Supplemental disclosures of non-cash investing and financing activities
Conversion of notes payable to common stock		10,813
Allowance for tenant improvements	653		586
Amounts accrued for patent expenditures	$ 11		$ 21