Income Taxes (Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Property and leasehold costs	$ 488,584	$ 572,481
Liabilities for dismantlements	98,444	170,946
Postretirement and other employee benefits	134,444	214,288
Alternative minimum tax	29,710	29,710
Foreign tax credit carryforwards	228,159	33,295
U.S. net operating loss	272,930	454,231
Other deferred tax assets	13,892	16,541
Total gross deferred tax assets	1,266,163	1,491,492
Less valuation allowance	(476,256)	(305,389)
Net deferred tax assets	789,907	1,186,103
Deferred tax liabilities
Deferred tax on undistributed foreign earnings	(65,000)
Accumulated depreciation, depletion and amortization	(669,638)	(867,343)
Other deferred tax liabilities	(2,824)	(21,908)
Total gross deferred tax liabilities	(737,462)	(889,251)
Net deferred tax assets	$ 52,445	$ 296,852