Condensed Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Cash flows from operating activities:
Net loss	$ (10,854)	$ (10,645)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation of property and equipment	314	202
Loss on disposal of property and equipment	25
Stock-based compensation	955	1,035
Amortization of deferred license fees	(1,036)	(1,036)
Warrant licensing expense		27
Amortization of premium on short-term investments	140	104
Changes in assets and liabilities:
Prepaid expenses and other current assets	(474)	(884)
Accounts payable	(1,571)	184
Accrued expenses	372	(195)
Accrued interest	5	5
Net cash used in operating activities	(12,124)	(11,203)
Cash flows from investing activities:
Purchases of marketable securities	(9,231)	(29,963)
Maturities of marketable securities	16,431	16,230
Purchases of property and equipment	(376)	(195)
Net cash provided by (used in) investing activities	6,824	(13,928)
Cash flows from financing activities:
Proceeds from exercise of common stock options and stock purchase plan	90	1,642
Payments on capital lease obligations	(82)
Net cash provided by financing activities	8	1,642
Net decrease in cash and cash equivalents	(5,292)	(23,489)
Cash and cash equivalents, beginning of period	13,345	35,781
Cash and cash equivalents, end of period	8,053	12,292
Supplemental disclosure of non-cash investing and financing activities:
Unrealized gain (loss) on available-for-sale investments	(7)	35
Issuance of 30,534 and 32,872 shares of common stock to fund the Company's 401(k) matching contribution for 2012 and 2011, respectively	$ 354	$ 274