Sale of Accounts Receivable (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Continuing Involvement with Derecognized Transferred Financial Assets, Amount Outstanding	$ 1,800		$ 1,800		$ 1,600
Proceeds from Sale and Collection of Receivables			24,300	$ 24,400
Collections applied to deferred consideration			24,300	23,900
Proceeds from transfer of receivables	1,400		1,400		1,000
Deferred receivables consideration	399		399		$ 540
Loss on transfer of accounts receivables to purchasers	2	$ 2	7	$ 4
Accounts Receivable Securitization Facility [Member]
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,700		1,700
Purchasers [Member] | Accounts Receivable Securitization Facility [Member]
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,200		1,200
Second Purchasers [Member] | Accounts Receivable Securitization Facility [Member]
Securitization or Asset-backed Financing Arrangement, Financial Asset for which Transfer is Accounted as Sale [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 500		$ 500