CONDENSED CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Statement Of Financial Position [Abstract]
Trade accounts receivable, less allowance for doubtful accounts	$ 22,476	$ 19,180
Property, plant and equipment, less allowance for depreciation	$ 773,375	$ 759,640
Preferred stock, par value	$ 1	$ 1
Preferred stock, shares authorized	10,000,000	10,000,000
Preferred stock, shares issued	0	0
Common stock, par value	$ 1	$ 1
Common stock, shares authorized	450,000,000	450,000,000
Common stock, shares issued	154,966,430	154,841,438