Schedule II - Qualifying and Valuation Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Year	$ 3,773	$ 4,110
Acquisitions and Dispositions	(21)	(365)
Charged to Costs and Expenses	3,054	2,849
Write-offs	(2,936)	(2,821)
Balance at End of Year	$ 3,870	$ 3,773