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Equipment Installment Plans - Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	$ 1,022	$ 1,007
2018	1
2019	91
2020	390
2021	540
Unbilled | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	962	948
Billed | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	42	43
Billed | Past Due
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	18	16
Lowest Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	886	863
2018	1
2019	84
2020	346
2021	455
Lowest Risk | Unbilled | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	840	819
Lowest Risk | Billed | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	36	36
Lowest Risk | Billed | Past Due
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	10	8
Lower Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	103	108
2018	0
2019	6
2020	36
2021	61
Lower Risk | Unbilled | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	93	98
Lower Risk | Billed | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	5	5
Lower Risk | Billed | Past Due
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	5	5
Slight Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	26	25
2018	0
2019	1
2020	6
2021	19
Slight Risk | Unbilled | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	23	22
Slight Risk | Billed | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	1	1
Slight Risk | Billed | Past Due
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	2	2
Higher Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	7	11
2018	0
2019	0
2020	2
2021	5
Higher Risk | Unbilled | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	6	9
Higher Risk | Billed | Current
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	0	1
Higher Risk | Billed | Past Due
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	$ 1	$ 1

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