Equipment Installment Plans - Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	$ 919	$ 873
Unbilled
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	869	827
Billed
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	33	32
Equipment installment plan receivables, past due	17	14
Lower Risk
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	895	850
Lower Risk \| Unbilled
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	848	807
Lower Risk \| Billed
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	32	31
Equipment installment plan receivables, past due	15	12
Higher Risk
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	24	23
Higher Risk \| Unbilled
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	21	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://fasb.org/us-gaap/role/ref/legacyRef
-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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.10(3))
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