Other Financial Information (Noncash Opearting Working Capital Increase (Decrease)) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Financial Information [Abstract]
Accounts receivable	$ 114,401	$ 119,671	$ 297,625
Inventories	26,883	(5,171)	(15,340)
Prepaid expenses	29,570	149,946	(144,845)
Deferred income tax assets			3,924
Accounts payable and accrued liabilities	(51,439)	(328,078)	(36,887)
Current income tax liabilities	16,999	24,943	(69,413)
Net (increase) decrease in noncash operating working capital	136,414	(38,689)	35,064
Cash income taxes paid, net of refunds	68,076	6,707	118,667
Interest paid, net of amounts capitalized	147,975	127,798	110,386
Asset retirement costs capitalized	8,509	13,690	76,775
Decrease in capital expenditure accrual	99,199	158,885	462,474
Interest paid, amounts capitalized	$ 4,488	$ 4,322	$ 7,290