Debt (Short Term Debt Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Short-term debt	$ 9,440,000,000	$ 7,790,000,000
Line of credit | Accounts Receivable Securitization Facility
Short-term Debt [Line Items]
Short-term debt	695,000,000	$ 568,000,000
Average outstanding amount	$ 605,000,000
Interest rate during period	1.90%
Maximum month-end outstanding amount	$ 695,000,000
Line of credit facility, maximum borrowing capacity	$ 775,000,000
Extension period	364 days
Collateral amount	$ 905,000,000