Preferred Stock - Voting and Dividends (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 item	Dec. 31, 2016 USD ($)
Series B and Series C convertible preferred stock
Preferred Stock
Number of board members that shareholders have exclusive right to elect | item	3
Series A Redeemable Convertible Preferred Stock
Preferred Stock
Cumulative unpaid dividends		$ 2,800
Series B Convertible Preferred Stock
Preferred Stock
Dividend rate (as a percent)	8.00%
Cumulative unpaid dividends		6,689
Series C Convertible Preferred Stock
Preferred Stock
Dividend rate (as a percent)	8.00%
Cumulative unpaid dividends		$ 2,383