Other Financial Information (Noncash Opearting Working Capital Increase (Decrease)) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other Financial Information [Abstract]
Decrease in accounts receivable	$ 30,467	$ 36,246
Decrease in inventories	12,189	4,497
Decrease (increase) in prepaid expenses	(5,031)	17,788
Decrease in accounts payable and accrued liabilities	(25,270)	(44,240)
Increase in income taxes payable	29,199	29,127
Net decrease in noncash operating working capital	41,554	43,418
Cash income taxes paid, net of refunds	7,940	3,422
Interest paid, net of amounts capitalized of $1,139 in 2018 and $1,093 in 2017	35,666	17,720
Asset retirement costs capitalized	727	565
Decrease (increase) in capital expenditure accrual	(10,757)	12,906
Interest paid, amounts capitalized	$ 1,139	$ 1,093