CONDENSED STATEMENTS OF CASH FLOWS CHANGE IN CASH AND CASH EQUIVALENTS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Cash flows from operating activities:
Net loss	$ (8,440)	$ (11,897)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	15	142
Accretion and amortization on investments, net	606	328
Gain on sales of property and equipment, net		(104)
Stock-based compensation for services by non-employees	65	24
Stock-based compensation for employees and directors	1,641	1,348
Amortization related to 401(k) contributions	12	103
Unrealized gain on derivatives	(224)	(25)
Changes in assets and liabilities:
Other current and noncurrent assets	(646)	345
Other current liabilities	(2,534)	(6,491)
Net cash used in operating activities	(9,505)	(16,227)
Cash flows from investing activities:
Purchases of property and equipment		(34)
Proceeds from sales of property and equipment		104
Purchases of marketable securities	(107,144)	(18,007)
Proceeds from maturities of marketable securities	17,363	27,595
Net cash (used in) provided by investing activities	(89,781)	9,658
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	97,620	11
Net cash provided by financing activities	97,620	11
Net decrease in cash and cash equivalents	(1,666)	(6,558)
Cash and cash equivalents at the beginning of the period	12,990	22,063
Cash and cash equivalents at the end of the period	$ 11,324	$ 15,505