Equipment Installment Plans - Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	$ 951	$ 1,008
2017	1
2018	93
2019	389
2020	468
Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	894	942
Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	40	45
Equipment installment plan receivables, past due	17	21
Lowest Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	809	860
2017	1
2018	84
2019	337
2020	387
Lowest Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	767	812
Lowest Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	34	37
Equipment installment plan receivables, past due	8	11
Lower Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	104	110
2017	0
2018	7
2019	39
2020	58
Lower Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	95	99
Lower Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	4	5
Equipment installment plan receivables, past due	5	6
Slight Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	25	28
2017	0
2018	2
2019	9
2020	14
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	13	10
2017	0
2018	0
2019	4
2020	9
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	$ 2	$ 1