Income Taxes (Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Deferred tax assets
Property and leasehold costs	$ 379,577	$ 708,947
Liabilities for dismantlements	85,544	79,111
Postretirement and other employee benefits	208,600	175,446
Alternative minimum tax	46,792	49,536
Foreign tax credit carryforwards	44,061	19,896
Other deferred tax assets	22,426	23,352
Total gross deferred tax assets	787,000	1,056,288
Less valuation allowance	(306,463)	(633,735)
Net deferred tax assets	480,537	422,553
Deferred tax liabilities
Property, plant and equipment	(479,677)	(747,561)
Accumulated depreciation, depletion and amortization	(1,123,864)	(1,040,251)
Other deferred tax liabilities	(15,753)	(38,849)
Total gross deferred tax liabilities	(1,619,294)	(1,826,661)
Net deferred tax liabilities	$ (1,138,757)	$ (1,404,108)