CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]
Accounts receivable, allowance for doubtful accounts	$ 1,609	$ 1,609
Property, plant and equipment, accumulated depreciation, depletion and amortization	$ 8,850,951	$ 8,540,239
Cumulative Preferred Stock, par	$ 100	$ 100
Cumulative Preferred Stock, authorized	400,000	400,000
Cumulative Preferred Stock, issued
Common Stock, par	$ 1.00	$ 1.00
Common Stock, authorized	450,000,000	450,000,000
Common Stock, issued	194,945,904	194,920,155
Treasury stock, shares of Common Stock	15,499,120	11,513,642