CONDENSED STATEMENTS OF CASH FLOWS CHANGE IN CASH AND CASH EQUIVALENTS (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Cash flows from operating activities:
Net loss	$ (9,315)	$ (8,440)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	9	15
Accretion and amortization on investments, net	629	606
Stock-based compensation for services by non-employees	67	65
Stock-based compensation for employees and directors	2,013	1,641
Amortization related to 401(k) contributions	154	12
Unrealized gain on derivatives	(16)	(224)
Changes in assets and liabilities:
Other current and noncurrent assets	(95)	(646)
Other current liabilities	(1,151)	(2,534)
Net cash used in operating activities	(7,705)	(9,505)
Cash flows from investing activities:
Purchases of property and equipment	(7)
Purchases of marketable securities	(78,869)	(107,144)
Proceeds from maturities of marketable securities	52,490	17,363
Net cash used in investing activities	(26,386)	(89,781)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of issuance costs	1,477	97,620
Net cash provided by financing activities	1,477	97,620
Net decrease in cash and cash equivalents	(32,614)	(1,666)
Cash and cash equivalents at the beginning of the period	42,796	12,990
Cash and cash equivalents at the end of the period	$ 10,182	$ 11,324