CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Receivables allowance	$ 1,034	$ 1,119
Accrued rental income allowance	1,842	1,936
Debt costs accumulated amortization	$ 13,734	$ 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)	158,933,104	153,577,028
Common stock, shares outstanding (in shares)	158,933,104	153,577,028
Series E Preferred Stock
Dividend rate	5.70%	5.70%
Preferred stock, depositary shares issued (in shares)	115,000	115,000
Preferred stock, depositary shares outstanding (in shares)	115,000	115,000
Preferred stock, stated liquidation value per share (in dollars per share)	$ 2,500	$ 2,500
Series F Preferred Stock
Dividend rate	5.20%	5.20%
Preferred stock, depositary shares issued (in shares)	138,000	138,000
Preferred stock, depositary shares outstanding (in shares)	138,000	138,000
Preferred stock, stated liquidation value per share (in dollars per share)	$ 2,500	$ 2,500