Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Accumulated depreciation on operating properties	$ 459,423	$ 421,495
Accumulated amortization on intangible liabilities	$ 45,545	$ 42,473
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	430,926,519	430,724,520
Treasury stock, shares	10,339,735	10,339,735