Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Accumulated depreciation on operating properties	$ 791,431	$ 803,073
Common stock, par value (in usd per share)	$ 1	$ 1
Common stock, shares authorized (in shares)	300,000,000	300,000,000
Common stock, shares issued (in shares)	151,272,969	151,149,289
Common stock, shares outstanding (in shares)	151,272,969	151,149,289
Treasury stock, shares (in shares)	2,584,933	2,584,933