CONSOLIDATED BALANCE SHEETS (unaudited) (Parenthetical) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,605	$ 1,605
Property, plant and equipment, accumulated depreciation, depletion and amortization	$ 10,285,996	$ 9,333,646
Cumulative preferred stock, par value (in USD per share)	$ 100	$ 100
Cumulative preferred stock, authorized shares (in shares)	400,000	400,000
Cumulative preferred stock, shares issued (in shares)	0	0
Common stock, par value (in USD per share)	$ 1	$ 1
Common stock, authorized shares (in shares)	450,000,000	450,000,000
Common stock, shares issued (in shares)	195,100,628	195,089,269