Common Stock (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended
	Jul. 31, 2013	Sep. 30, 2013	Dec. 31, 2012
Common Stock
Shares of common stock sold in underwritten public offering	5,750,000
Public offering price (in dollars per share)	$ 5.00
Net proceeds from sale of common stock and convertible senior notes	$ 26.7
Common stock, authorized shares		200,000,000	200,000,000
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock, issued shares		102,275,000	95,825,000
Common stock, outstanding shares		102,275,000	95,825,000
Shares reserved for future issuance
Common stock warrants (in shares)		2,777,000	2,842,000
Options to purchase common stock (in shares)		20,355,000	18,066,000
Conversion premium on the Notes (in shares)		25,000,000
Shares reserved for future issuance		48,132,000	20,908,000