Equipment Installment Plan Receivables, Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	$ 871	$ 873
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	828	827
Billed
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	29	32
Equipment installment plan receivables, past due	14	14
Lower Risk
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	849	850
Lower Risk \| Unbilled
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	809	807
Lower Risk \| Billed
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	28	31
Equipment installment plan receivables, past due	12	12
Higher Risk
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	22	23
Higher Risk \| Unbilled
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	19	20
Higher Risk \| Billed
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	1	1
Equipment installment plan receivables, past due	$ 2	$ 2

Financing receivables that are current.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7A
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953401-111524

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Recorded Investment in financing receivables that are past due at the balance sheet date.

+ References

+ Details

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

- Definition

Amount representing an agreement for an unconditional promise by the maker to pay the entity (holder) a definite sum of money at a future date. Such amount may include accrued interest receivable in accordance with the terms of the note. The note also may contain provisions and related items including a discount or premium, payable on demand, secured, or unsecured, interest bearing or noninterest bearing, among a myriad of other features and characteristics. Excludes amounts related to receivables held-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis=us-gaap_UnbilledRevenuesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis=us-gaap_BilledRevenuesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_PrimeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InternalCreditAssessmentAxis=us-gaap_SubprimeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: