CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Statement Of Financial Position [Abstract]
Maturity of Canadian government securities	90 days
Accounts receivable, allowance for doubtful accounts	$ 1,609	$ 1,609
Property, plant and equipment, accumulated depreciation, depletion and amortization	$ 9,503,524	$ 8,540,239
Cumulative Preferred Stock, par	$ 100	$ 100
Cumulative Preferred Stock, authorized	400,000	400,000
Cumulative Preferred Stock, issued	0	0
Common Stock, par	$ 1.00	$ 1.00
Common Stock, authorized	450,000,000	450,000,000
Common Stock, issued	195,040,149	194,920,155