Sale of Accounts Receivable (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounts Receivable Securitization Programs [Line Items]
Proceeds from transfer of receivables	$ 1,400,000,000	$ 1,000,000,000
Deferred receivables consideration	307,000,000	540,000,000
Proceeds from the collection of receivables sold	33,500,000,000	34,300,000,000	$ 40,700,000,000
Collections applied to deferred consideration	33,500,000,000	33,800,000,000	40,300,000,000
Receivables sold under securitization facility derecognized during the period	1,700,000,000	1,600,000,000
Loss on transfer of accounts receivables to purchasers	10,000,000	$ 6,000,000	$ 5,000,000
Lines of credit	7,700,000,000
Accounts Receivable Securitization Facility [Member]
Accounts Receivable Securitization Programs [Line Items]
Lines of credit	1,600,000,000
Program [Member] | Accounts Receivable Securitization Facility [Member]
Accounts Receivable Securitization Programs [Line Items]
Lines of credit	1,200,000,000.0
Second Program [Member] | Accounts Receivable Securitization Facility [Member]
Accounts Receivable Securitization Programs [Line Items]
Lines of credit	$ 400,000,000