Income Taxes Income Taxes - Operating Loss and Tax Credit Carryforwards (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	$ 1,213	$ 1,287
Deferred Tax Assets, Tax Credit Carryforwards	177	165
Total Operating Loss and Tax Credit Carryforwards	1,390	1,452
Expiring within Five Years [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	76	42
Deferred Tax Assets, Tax Credit Carryforwards	8	10
Expiring After Five Years Or Having Indefinite Expiration [Member]
Operating Loss Carryforwards [Line Items]
Deferred Tax Assets, Operating Loss Carryforwards	1,137	1,245
Deferred Tax Assets, Tax Credit Carryforwards	$ 169	$ 155