Income Taxes - Future Tax Benefits (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Net operating loss carryover, Expected tax benefit, gross	$ 710	$ 1,458
Foreign tax credit credit carryover, Expected tax benefit, gross	26	56
Alternative minimum tax credit carryover, Expected tax benefit, gross	0	640
General business credit carryover, Expected tax benefit, gross	3	$ 99
Foreign tax credit carryover | Expiring within Tax Years 2021 to 2024
Tax Credit Carryforward [Line Items]
Tax credit carryover, Carryover amount	26
Foreign tax credit credit carryover, Expected tax benefit, gross	26
Alternative minimum tax credit carryover
Tax Credit Carryforward [Line Items]
Tax credit carryover, Carryover amount	790
Alternative minimum tax credit carryover, Expected tax benefit, gross	790
Alternative minimum tax credit carryover | Expiring within Tax Years 2031 to 2035
Tax Credit Carryforward [Line Items]
Alternative minimum tax credit carryover, Expected tax benefit, gross	0
General business credit carryover
Tax Credit Carryforward [Line Items]
Tax credit carryover, Carryover amount	3
General business credit carryover, Expected tax benefit, gross	3
General business credit carryover | Expiring within Tax Years 2031 to 2035
Tax Credit Carryforward [Line Items]
General business credit carryover, Expected tax benefit, gross	3
U.S.
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards	3,380
Net operating loss carryover, Expected tax benefit, gross	710
U.S. | Expiring Tax Year 2020
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards	1
U.S. | Expiring within Tax Years 2023 to 2036
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards	3,379
Foreign
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards	3
Net operating loss carryover, Expected tax benefit, gross	0
Foreign | No expiration tax year
Tax Credit Carryforward [Line Items]
Operating Loss Carryforwards	$ 3