Income Taxes (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Effective income tax rate		8.80%	32.60%	13.60%	27.10%
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%		35.00%
Effect of Accounting Standards Update 2018-02	[1]			$ (202)
Unrecognized Tax Benefits, Period Increase (Decrease)		$ 15		15
Unrecognized Tax Benefits, Income Tax Penalties and Interest Expense		$ 1		1
Reduction in income tax rate [Member]
Income Tax Contingency [Line Items]
Change in Deferred Tax Assets due to 2017 Tax Act						$ (252)
Change in Deferred Tax Liabilities due to 2017 Tax Act						(196)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount						56
Acceleration of Depreciation [Member]
Income Tax Contingency [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount						8
Change in Current Income Tax Payable due to 2017 Tax Act						$ 8
Executive Compensation [Member]
Income Tax Contingency [Line Items]
Compensation Tax Deduction Limitation				$ 1

[1] The Company adopted ASU 2018-02 as of January 1, 2018. Accordingly, stranded tax effects of $202 million related to the Tax Act were reclassified to retained earnings.