Stockholders' Equity - Common Stock and Warrants (Details) $ / shares in Units, $ in Thousands				1 Months Ended	2 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended	33 Months Ended
	Apr. 12, 2017 USD ($) $ / shares shares	Apr. 03, 2017 USD ($) $ / shares	Dec. 22, 2016 $ / shares shares	Mar. 31, 2019 USD ($) $ / shares shares	Aug. 31, 2019 USD ($)	Jun. 30, 2019 USD ($) $ / shares shares	Mar. 31, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) item $ / shares shares	Jun. 30, 2018 shares	Dec. 31, 2017 shares	Dec. 31, 2019 USD ($)	Dec. 31, 2018 $ / shares shares	Jul. 20, 2017 $ / shares
Stockholders' equity
Preferred stock, Shares authorized | shares						5,000,000		5,000,000
Preferred stock, par value | $ / shares						$ 0.01		$ 0.01
Common Stock Shares, Outstanding | shares						231,954,736		231,954,736				219,157,998
Common stock, par value (in dollars per share) | $ / shares						$ 0.01		$ 0.01				$ 0.01
Number of votes per share | item								1
Common stock shares issued | shares				10,000,000
Par value, common stock | $ / shares						0.01		$ 0.01				0.01
Share price (in dollars per share) | $ / shares				$ 2.35			$ 2.35
Proceeds from public offerings, net of transaction costs | $				$ 23,500				$ 28,265
Exercise price of warrants (in dollars per share) | $ / shares													$ 2.1231
Value of shares issued | $								$ 28,265
Series A Junior Participating Cumulative Preferred Stock
Stockholders' equity
Preferred stock, par value | $ / shares						$ 0.01		$ 0.01				$ 0.01
Common Stock Shares, Outstanding | shares						0		0				0
At Market Issuance Sales Agreement
Stockholders' equity
Common stock, par value (in dollars per share) | $ / shares		$ 0.01
Common stock shares issued | shares						2,100,000
Par value, common stock | $ / shares		$ 0.01
Proceeds from public offerings, net of transaction costs | $						$ 5,500
Authorized amount | $		$ 75,000
Value of shares issued | $					$ 8,800	$ 5,500	$ 0				$ 45,800
Warrants issued in December 2016
Stockholders' equity
Class of Warrant or Right Issued | shares			10,501,500
Exercise price of warrants (in dollars per share) | $ / shares			$ 1.50
Number of warrants exercised (in shares) | shares	10,501,500
Proceeds from exercise of warrants, net of transaction costs | $	$ 15,100
Shares of common stock that can be purchased from warrants issued | shares	5,250,750
Per share price of shares of common stock | $ / shares	$ 2.69
Warrant Issued With Amazon And Walmart Stores Inc Transaction Agreement In 2017
Stockholders' equity
Number of warrants exercised (in shares) | shares								26,188,434	18,913,869
Warrant Issued With Amazon And Walmart Stores Inc Transaction Agreement In 2017 | Maximum
Stockholders' equity
Class of Warrant or Right Issued | shares										110,573,392