CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Allowance on receivables	$ 1,119	$ 1,006
Allowance on accrued rental income	1,936	3,078
Accumulated amortization	$ 12,667	$ 11,268
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	15,000,000	15,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	375,000,000	375,000,000
Common stock, shares issued (in shares)	153,577,028	147,149,945
Common stock, shares outstanding (in shares)	153,577,028	147,149,945
Series D Preferred Stock
Preferred stock, issued (in shares)		115,000
Preferred stock, outstanding (in shares)		115,000
Stated liquidation value per share (in dollars per share)		$ 2,500
Dividend rate (percent)		6.625%
Series E Preferred Stock
Preferred stock, issued (in shares)	115,000	115,000
Preferred stock, outstanding (in shares)	115,000	115,000
Stated liquidation value per share (in dollars per share)	$ 2,500	$ 2,500
Dividend rate (percent)	5.70%	5.70%
Series F Preferred Stock
Preferred stock, issued (in shares)	138,000	138,000
Preferred stock, outstanding (in shares)	138,000	138,000
Stated liquidation value per share (in dollars per share)	$ 2,500	$ 2,500
Dividend rate (percent)	5.20%	5.20%