Income Taxes (Components of Net Deferred Tax Liability) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred Tax Assets:
Net operating loss carryforwards	$ 481	$ 544
Nondeductible accruals and other	2,026	3,797
Less: Valuation allowance	377	266
Total deferred tax assets, net of valuation allowance	2,130	4,075
Deferred Tax Liabilities:
Differences between book and tax basis of property and equipment and intangible assets	25,154	36,957
Differences between book and tax basis of investments	466	498
Differences between book and tax basis of long-term debt	673	571
Differences between book and tax basis of foreign subsidiaries and undistributed foreign earnings	39	862
Total deferred tax liabilities	26,332	38,888
Net deferred tax liability	$ 24,202	$ 34,813