STATEMENTS OF CASH FLOWS (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Activities:
Net loss	$ (28,507)	$ (19,364)	$ (35,708)
Reconciliation of net loss to net cash used in operating activities:
Change in fair value of warrant liability	(101)	(2,474)	(1,671)
Depreciation and amortization	1,666	1,613	1,372
(Gain) Loss on disposal of property and equipment	(28)		35
Impairment of long-lived assets		360	162
Amortization of net premiums on investments	(18)	(317)	(247)
Deferred rent	660	(341)	(282)
Non-cash stock-based compensation	2,091	2,047	1,339
Realized gains on short-term investments	(879)
Changes in operating assets and liabilities:
Restricted cash	(986)
Accounts receivables	954	(1,911)	204
Unbilled receivables	266	(1,836)
Prepaid expenses and other assets	40	(1,854)	(312)
Accounts payable and accrued expenses	2,009	(2,686)	1,912
Deferred revenue	258	2,500	(150)
Lease incentives received	4,346
Net cash used in operating activities	(18,229)	(23,629)	(32,852)
Investing Activities:
Capital expenditures	(4,341)	(610)	(1,556)
Proceeds from disposal of property and equipment	324
Purchases of investments	(48,652)	(2,082)	(38,717)
Proceeds from maturities and redemptions of investments	20,407	21,235	19,000
Net cash (used in) provided by investing activities	(32,262)	18,543	(21,273)
Financing Activities:
Principal payments of capital lease	(104)
Principal payments of notes payable	(60)	(80)	(86)
Proceeds from notes payable	650
Proceeds from settlement of notes receivable from former directors		50
Restricted cash	(756)
Net proceeds from sales of common stock, net of offering costs of $0.4 million, $0.2 million and $0.5 million, respectively	54,002	10,980	23,089
Proceeds from the exercise of stock options	54	179	426
Net cash provided by financing activities	53,786	11,129	23,429
Net (decrease) increase in cash and cash equivalents	3,295	6,043	(30,696)
Cash and cash equivalents at beginning of period	14,104	8,061	38,757
Cash and cash equivalents at end of period	17,399	14,104	8,061
Supplemental disclosure of non-cash activities:
Deposit applied towards the purchase of equipment	500
Equipment acquired under a capital lease	399
Equipment purchases included in accounts payable and accrued expenses	1,321	14	418
Settlement of notes receivable from former directors		1,522
Sale of common stock under the 2010 Sales Agreement not settled at year-end		847
Supplemental disclosure of cash flow information:
Cash interest payments	$ 20