Common Stock (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012
Common Stock
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, authorized shares	200,000,000	200,000,000
Common stock, issued shares	95,948,000	95,825,000
Common stock, outstanding shares	95,948,000	95,825,000
Shares reserved for future issuance
Common stock warrants (in shares)	2,779,000	2,842,000
Options to purchase common stock	20,664,000	18,066,000
Shares reserved for future issuance	23,443,000	20,908,000