Income Taxes (Tax Effects of Temporary Differences and Carryforwards of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Abstract]
Loss and tax credit carryforwards	$ 479	$ 652
Other assets	45	43
Asset impairments and restructuring reserves	33	94
Postretirement medical and life benefits	162	191
Other accrued liabilities	265
Other employee benefits	289	278
Gross deferred tax assets	1,273	1,258
Valuation allowance	(317)	(456)
Total deferred tax assets	956	802
Intangible and other assets	(96)	(101)
Other accrued liabilities		(94)
Fixed assets	(256)	(245)
Total deferred tax liabilities	(352)	(440)
Net deferred tax assets	$ 604	$ 362