Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013
Operating activities
Net loss	$ (34,512)	$ (16,741)
Adjustments to reconcile net loss to net cash used in operating activities
Depreciation and amortization expense	1,098	996
(Gain) loss from valuation of convertible note payable	(614)	3,787
Stock-based compensation	4,710	2,524
Amortization of premium on investments, net	1,235	1,012
Loss on disposal of long-term assets	18
Change in operating assets and liabilities:
Contracts and other receivables	50	(95)
Prepaid and other assets	(882)	(512)
Accounts payable	1,470	829
Accrued liabilities	610	851
Accrued compensation	147	(120)
Deferred revenue	(980)	(11,615)
Deferred rent and other liabilities	(203)	68
Net cash used in operating activities	(27,853)	(19,016)
Investing activities
Purchases of short-term investments	(52,268)	(50,390)
Maturities and sales of short-term investments	63,074	16,520
Purchases of property and equipment	(1,120)	(608)
Acquisition of intangibles	(53)	(46)
Net cash provided by (used in) investing activities	9,633	(34,524)
Financing activities
Proceeds from issuance of common stock, net	9,853	46,562
Proceeds from exercise of common stock options	624
Principal payments on other long-term obligations	(106)	(84)
Net cash provided by financing activities	10,371	46,478
Net decrease in cash and cash equivalents	(7,849)	(7,062)
Cash and cash equivalents at beginning of period	17,807	40,552
Cash and cash equivalents at end of period	9,958	33,490
Supplemental disclosure of cash flow information
Interest paid	30
Income taxes paid	1	1
Supplemental disclosure of non-cash investing and financing activities
Amounts accrued for property and equipment, net		57
Allowance for tenant improvements		$ 947