Income Taxes (Tax Effects of Temporary Differences and Carryforwards of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Abstract]
Loss and tax credit carryforwards	$ 388	$ 479
Other assets	345	45
Asset impairments and restructuring reserves	30	33
Postretirement medical and life benefits	168	162
Other accrued liabilities	218	265
Other employee benefits	345	289
Gross deferred tax assets	1,494	1,273
Valuation allowance	(215)	(317)
Total deferred tax assets	1,279	956
Intangible and other assets	(110)	(96)
Fixed assets	(216)	(256)
Financing leases	(121)
Total deferred tax liabilities	(447)	(352)
Net deferred tax assets	$ 832	$ 604