Sale Of Equipment Installment Receivables (Derecognized Installment Receivables) (Details) $ in Millions	Mar. 31, 2018 USD ($)
Derecognized Installment Receivables [Abstract]
Outstanding derecognized receivables at January 1,	$ 7,446
Outstanding derecognized receivables at March 31,	$ 8,895

Amount of transferred financial assets in which the transferor has continuing involvement with the assets underlying the transaction that have been derecognized. Continuing involvement includes, but is not limited to, servicing, recourse, and restrictions on transferor's interests in transferred financial assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

Name:	us-gaap_DerecognizedAssetsSecuritizedOrAssetbackedFinancingArrangementAssetsAndAnyOtherFinancialAssetsManagedTogetherPrincipalAmountOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant