Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Statement of Financial Position [Abstract]
Accumulated depreciation on operating properties	$ 384,097	$ 275,977
Allowance for doubtful accounts for notes and accounts receivable	456	535
Accumulated amortization on intangible liabilities	$ 39,332	$ 28,960
Preferred stock, par value (in usd per share)	$ 1	$ 1
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock, shares issued	6,867,357	6,867,357
Preferred stock, shares outstanding	6,867,357	6,867,357
Common stock, par value (in usd per share)	$ 1	$ 1
Common stock, shares authorized	700,000,000	700,000,000
Common stock, shares issued	430,724,520	430,349,620
Treasury stock, shares	10,339,735	10,329,082