Equipment Installment Plans - Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	$ 1,013	$ 1,007
2018	17
2019	224
2020	562
2021	210
Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	957	948
Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	42	43
Equipment installment plan receivables, past due	14	16
Lowest Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	868	863
2018	15
2019	202
2020	484
2021	167
Lowest Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	826	819
Lowest Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	35	36
Equipment installment plan receivables, past due	7	8
Lower Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	108	108
2018	1
2019	18
2020	61
2021	28
Lower Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	99	98
Lower Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	5	5
Equipment installment plan receivables, past due	4	5
Slight Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	27	25
2018	1
2019	3
2020	12
2021	11
Slight Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	24	22
Slight Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	1	1
Equipment installment plan receivables, past due	2	2
Higher Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	10	11
2018	0
2019	1
2020	5
2021	4
Higher Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	8	9
Higher Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	1	1
Equipment installment plan receivables, past due	$ 1	$ 1