Sales of Common Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended			12 Months Ended			34 Months Ended
	Oct. 31, 2012	May 31, 2012	Jul. 31, 2008	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012	Mar. 31, 2010
Issuance of common stock, shares	12,385,321	10,000,000	6,686,650	8,442,025	6,001,841	10,513,849	24,957,715
Underlying price of common stock per share	$ 2.18	$ 1.22		$ 1.89	$ 1.26		$ 1.89
Proceeds from direct offering	$ 27,000	$ 12,200	$ 17,500	$ 14,000	$ 11,800	$ 23,100	$ 49,900
Gross proceeds authorized in sale of stock	50,000
Gross proceeds authorized in sale of stock, prior sales agreement								$ 50,000
Minimum [Member]
Underlying price of common stock per share				$ 1.31	$ 1.25	$ 2.10	$ 1.31
Maximum [Member]
Underlying price of common stock per share				$ 2.18	$ 2.75	$ 2.55	$ 2.18