Condensed Consolidated Balance Sheets (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Current Assets:
Cash and cash equivalents	$ 49,101	$ 256,965
Short-term investments	176,091
Accounts receivable	7,849	3,530
Restricted investments	7,250	7,250
Other current assets	9,079	3,061
Total Current Assets	249,370	270,806
Restricted investments, noncurrent	647	647
Property and equipment, net of accumulated depreciation and amortization	20,130	15,049
Patent costs, net of accumulated amortization of $1,740 and $1,622	5,217	5,042
Other assets	25	25
Total Assets	275,389	291,569
Current Liabilities:
Accounts payable	7,721	8,080
Accrued expenses	16,340	14,601
Current portion of long-term debt	94	92
Warrant liability	9,213	9,006
Deferred revenue	3,303	3,299
Other liabilities	1,087	888
Total Current Liabilities	37,758	35,966
Long-term debt	1,551	1,576
Deferred rent and other long-term liabilities	7,118	6,835
Total Liabilities	46,427	44,377
Commitments and contingencies
Stockholders' Equity:
Preferred stock, $.0001 par value, 3,333,333 shares authorized; none issued and outstanding
Common stock, $.0001 par value, 50,000,000 shares authorized; 37,986,041 and 37,751,920 issued and outstanding at March 31, 2014 and December 31, 2013, respectively	4	4
Additional paid-in capital	800,526	790,424
Accumulated other comprehensive loss	(59)
Deficit accumulated during the development stage	(571,509)	(543,236)
Total Stockholders' Equity	228,962	247,192
Total Liabilities and Stockholders' Equity	$ 275,389	$ 291,569