Income Taxes - Analysis of Deferred Tax Assets and Deferred Tax Liabilities Showing Tax Effects of Significant Temporary Differences (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Property and leasehold costs	$ 233,351	$ 231,389
Liabilities for dismantlements	78,361	88,075
Postretirement and other employee benefits	125,250	113,826
Alternative minimum tax	9,765	9,765	$ 29,700
U. S. net operating loss	495,252	496,629
Other deferred tax assets	66,795	19,974
Total gross deferred tax assets	1,008,774	959,658
Less valuation allowance	(103,113)	(166,991)
Net deferred tax assets	905,661	792,667
Deferred tax liabilities
Deferred tax on undistributed foreign earnings	(5,000)	(5,000)	$ (65,000)
Accumulated depreciation, depletion and amortization	(938,614)	(710,384)
Deferred Tax Liabilities, Investments	(14,250)	(32,178)
Other deferred tax liabilities	(25,708)	(28,802)
Total gross deferred tax liabilities	(983,572)	(776,364)
Net deferred tax (liabilities) assets	$ (77,911)
Net deferred tax (liabilities) assets		$ 16,303