Preferred Stock (Series D Preferred Stock) (Narrative) (Details) - USD ($) $ / shares in Units, $ in Thousands			3 Months Ended	12 Months Ended
	Feb. 28, 2017	Mar. 16, 2016	Mar. 31, 2018	Dec. 31, 2017
Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding	6,245,156
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares outstanding		3,000,000
Preferred stock, annual dividend rate		6.25%
Series E Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, value	$ 9,900		$ 10,050	$ 10,050
Preferred stock, shares outstanding	925,000		925,000	925,000
Preferred stock, annual dividend rate	6.25%		6.25%	6.25%
Redeemable preferred stock, shares issued	925,000
Common Stock [Member]
Class of Stock [Line Items]
Shares issued upon conversion	6,004,957
Shares issued, price per share	$ 10.40
SREP III Flight Investco L.P. [Member] | Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued		3,000,000
Preferred stock, value		$ 30,000
Real Estate Strategies, L.P. [Member] | Series D Preferred Stock [Member]
Class of Stock [Line Items]
Preferred stock, shares issued		245,156
Shares issued upon conversion		3,000,000