Income Taxes (Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Property and leasehold costs	$ 491,660	$ 488,584
Liabilities for dismantlements	88,075	98,444
Postretirement and other employee benefits	113,826	134,444
Alternative minimum tax	9,765	29,710
Foreign tax credit carryforwards		228,159
U.S. net operating loss	496,629	272,930
Other deferred tax assets	19,974	13,892
Total gross deferred tax assets	1,219,929	1,266,163
Less valuation allowance	(213,815)	(476,256)
Net deferred tax assets	1,006,114	789,907
Deferred tax liabilities
Deferred tax on undistributed foreign earnings	(5,000)	(65,000)
Accumulated depreciation, depletion and amortization	(742,562)	(669,638)
Other deferred tax liabilities	(28,802)	(2,824)
Total gross deferred tax liabilities	(776,364)	(737,462)
Net deferred tax assets	$ 229,750	$ 52,445