CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Allowance on receivables	$ 2,273	$ 1,119
Allowance on accrued rental income	1,842	1,936
Accumulated amortization	$ 14,118	$ 12,667
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)	161,503,585	153,577,028
Common stock, shares outstanding (in shares)	161,503,585	153,577,028
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	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	5.20%	5.20%