Stockholders' Equity - Common Stock and Warrants (Details) - USD ($)				1 Months Ended					6 Months Ended		12 Months Ended
	Apr. 12, 2017	Apr. 03, 2017	Dec. 22, 2016	Feb. 28, 2018	Apr. 30, 2017	Dec. 31, 2016	Jan. 31, 2014	Feb. 28, 2013	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Jul. 20, 2017
Stockholders' equity
Preferred stock, Shares authorized									5,000,000
Preferred stock, par value									$ 0.01
Common Stock, par value (in dollars per share)									$ 0.01		$ 0.01
Common Stock Shares, Outstanding									214,653,162		228,486,366
Exercise price of warrants (in dollars per share)												$ 2.1231
Proceeds from exercise of warrants, net of transaction costs										$ 17,702,000
Series A Junior Participating Cumulative Preferred Stock
Stockholders' equity
Preferred stock, par value									$ 0.01
Common Stock Shares, Outstanding									0		0
At Market Issuance Sales Agreement
Stockholders' equity
Common Stock, par value (in dollars per share)		$ 0.01
Authorized amount		$ 75,000,000
Shares issued		$ 23,000,000
Warrants issued in February, 2013
Stockholders' equity
Class of Warrant or Right Issued								23,637,500
Exercise price of warrants (in dollars per share)				$ 0.15				$ 0.15
Warrants outstanding (in shares)									0		100
Number of warrants exercised (in shares)				100
Warrants issued in December 2016
Stockholders' equity
Class of Warrant or Right Issued			10,501,500			10,501,500
Exercise price of warrants (in dollars per share)			$ 1.50			$ 1.50
Number of warrants exercised (in shares)	10,501,500										10,501,500
Proceeds from exercise of warrants, net of transaction costs	$ 15.10
Shares of common stock that can be purchased from warrants issued	5,250,750
Per share price of shares of common stock	$ 2.69
Warrants issued in January, 2014
Stockholders' equity
Class of Warrant or Right Issued					4,000,000		4,000,000
Exercise price of warrants (in dollars per share)					$ 0.65	$ 0.65	$ 4.00
Number of warrants exercised (in shares)									4,000,000
Proceeds from exercise of warrants, net of transaction costs					$ 2,600,000
Increase in additional paid-in capital					27,100,000
Reduction of warrant liability					$ 27,100,000
Warrant Issued With Amazon And Walmart Stores Inc Transaction Agreement In 2017
Stockholders' equity
Class of Warrant or Right Issued											110,573,392
Number of warrants exercised (in shares)									18,913,869		18,913,869