CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Accounts receivable, allowance for doubtful accounts	$ 1,605	$ 1,605
Property, plant and equipment, accumulated depreciation, depletion and amortization	$ 12,027,902	$ 12,607,815
Cumulative Preferred Stock, par value (per share)	$ 100	$ 100
Cumulative Preferred Stock, authorized shares	400,000	400,000
Cumulative Preferred Stock, shares issued	0	0
Common Stock, par value (per share)	$ 1.00	$ 1.00
Common Stock, authorized shares	450,000,000	450,000,000
Common Stock, shares issued	195,055,724	195,055,724
Minimum [Member]
Maturity of Canadian government securities	90 days	90 days