Income Tax (Tax Credit Carryforwards) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 1,322	$ 1,375
Foreign Tax Authority
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	264
Foreign Tax Authority | 2018-2022
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	42
Foreign Tax Authority | 2023-2027
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	200
Foreign Tax Authority | 2028-2032
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1
Foreign Tax Authority | 2033-2037
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
Foreign Tax Authority | Indefinite
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	21
General business tax credit carryforward
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	1,068
General business tax credit carryforward | 2018-2022
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
General business tax credit carryforward | 2023-2027
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
General business tax credit carryforward | 2028-2032
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	236
General business tax credit carryforward | 2033-2037
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	832
General business tax credit carryforward | Indefinite
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
Other tax credit carryforward
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	263
Other tax credit carryforward | 2018-2022
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
Other tax credit carryforward | 2023-2027
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
Other tax credit carryforward | 2028-2032
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
Other tax credit carryforward | 2033-2037
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	0
Other tax credit carryforward | Indefinite
Tax Credit Carryforward [Line Items]
Tax credit carryforwards	$ 263