Equipment Installment Plans - Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	$ 948	$ 1,008
2017	2
2018	153
2019	476
2020	317
Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	890	942
Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	42	45
Equipment installment plan receivables, past due	16	21
Lowest Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	809	860
2017	2
2018	139
2019	408
2020	260
Lowest Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	764	812
Lowest Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	36	37
Equipment installment plan receivables, past due	9	11
Lower Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	102	110
2017	0
2018	11
2019	51
2020	40
Lower Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	94	99
Lower Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	4	5
Equipment installment plan receivables, past due	4	6
Slight Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	25	28
2017	0
2018	2
2019	12
2020	11
Slight Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	22	23
Slight Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	1	2
Equipment installment plan receivables, past due	2	3
Higher Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	12	10
2017	0
2018	1
2019	5
2020	6
Higher Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	10	8
Higher Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	1	1
Equipment installment plan receivables, past due	$ 1	$ 1