Income Taxes (Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets
Property and leasehold costs	$ 572,481	$ 587,517
Liabilities for dismantlements	170,946	114,565
Postretirement and other employee benefits	214,288	226,217
Alternative minimum tax	29,710	39,683
Foreign tax credit carryforwards	33,295	855
U.S. net operating loss	454,231
Other deferred tax assets	16,541	127,165
Total gross deferred tax assets	1,491,492	1,096,002
Less valuation allowance	(305,389)	(294,406)
Net deferred tax assets	1,186,103	801,596
Deferred tax liabilities
Accumulated depreciation, depletion and amortization	(867,343)	(793,972)
Other deferred tax liabilities	(21,908)	(21,095)
Total gross deferred tax liabilities	(889,251)	(815,067)
Net deferred tax assets (liabilities)	$ 296,852	$ (13,471)