CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Receivables allowance	$ 1,961	$ 1,006
Accrued rental income allowance	2,224	3,078
Debt costs accumulated amortization	$ 12,315	$ 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)	151,627,386	147,149,945
Common stock, shares outstanding (in shares)	151,627,386	147,149,945
Series D Preferred Stock
Dividend rate		6.625%
Preferred stock, depositary shares issued (in shares)		115,000
Preferred stock, depositary shares outstanding (in shares)		115,000
Preferred stock, stated liquidation value per share (in dollars per share)		$ 2,500
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