Asset Retirement Obligations (Schedule of Reconciliation of Beginning and Ending Aggregate Carrying Amount of Asset Retirement Obligations) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligations [Abstract]
Balance at beginning of year		$ 781,057	$ 825,312
Accretion expense		42,590	46,742	[1]	$ 48,665	[1]
Liabilities incurred		52,331	13,690
Revision of previous estimates		(47,612)	(4,511)
Liabilities settled		(29,111)	(20,589)
Liabilities assumed by purchaser of oil and gas assets		(87,456)	(91,883)
Changes due to translation of foreign currencies		10,340	12,296
Balance at end of year		$ 781,057	$ 825,312		$ 825,312		$ 722,139	$ 781,057
Liabilities reported as held for sale at end of year	[2]							(85,900)
Current portion of liability at end of year	[3]						(12,840)	(13,629)
Noncurrent portion of liability at end of year							$ 709,299	$ 681,528

[1] Reclassified to conform to current presentation (see Note A).

[2] Liabilities held for sale related to Seal properties in Canada which were sold in January 2017.

[3] Included in Other accrued liabilities on the Consolidated Balance Sheet.