Income Taxes (Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred tax assets
Property and leasehold costs	$ 587,517	$ 379,577
Liabilities for dismantlements	114,565	85,544
Postretirement and other employee benefits	226,217	208,600
Alternative minimum tax	39,683	46,792
Foreign tax credit carryforwards	855	44,061
Other deferred tax assets	127,165	22,426
Total gross deferred tax assets	1,096,002	787,000
Less valuation allowance	(294,406)	(306,463)
Net deferred tax assets	801,596	480,537
Deferred tax liabilities
Property, plant and equipment		(479,677)
Accumulated depreciation, depletion and amortization	(793,972)	(1,123,864)
Other deferred tax liabilities	(21,095)	(15,753)
Total gross deferred tax liabilities	(815,067)	(1,619,294)
Net deferred tax liabilities	$ (13,471)	$ (1,138,757)