CONDENSED STATEMENTS OF CASH FLOWS CHANGE IN CASH AND CASH EQUIVALENTS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014
Cash flows from operating activities:
Net income (loss)	$ 8,514	$ (26,723)
Adjustments to reconcile net income (loss) to net cash used in operating activities:
Depreciation and amortization	39	38
Accretion and amortization on investments, net	1,670	2,221
Loss on retirement of property and equipment		3
Stock-based compensation for services by non-employees	262	172
Stock-based compensation for employees and directors	6,453	5,752
Amortization related to 401(k) contributions	161	99
Recognition of deferred revenue	(35,000)
Unrealized gain on derivatives	(16)	(366)
Changes in assets and liabilities:
Other current and noncurrent assets	(370)	(484)
Other current liabilities	(658)	161
Net cash used in operating activities	(18,945)	(19,127)
Cash flows from investing activities:
Restricted cash transfer		530
Purchases of property and equipment	(49)	(12)
Purchases of marketable securities	(153,073)	(159,172)
Proceeds from sales and maturities of marketable securities	144,329	74,561
Net cash used in investing activities	(8,793)	(84,093)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	1,765	97,830
Net cash provided by financing activities	1,765	97,830
Net decrease in cash and cash equivalents	(25,973)	(5,390)
Cash and cash equivalents at the beginning of the period	42,796	12,990
Cash and cash equivalents at the end of the period	$ 16,823	$ 7,600