Equipment Installment Plans - Gross Receivables by Credit Category (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	$ 964	$ 1,008
2017	16
2018	223
2019	569
2020	156
Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	900	942
Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	43	45
Equipment installment plan receivables, past due	21	21
Lowest Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	820	860
2017	15
2018	200
2019	480
2020	125
Lowest Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	774	812
Lowest Risk | Billed
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	35	37
Equipment installment plan receivables, past due	11	11
Lower Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	106	110
2017	1
2018	19
2019	67
2020	19
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	5	5
Equipment installment plan receivables, past due	6	6
Slight Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	27	28
2017	0
2018	3
2019	16
2020	8
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	2	2
Equipment installment plan receivables, past due	3	3
Higher Risk
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables, gross	11	10
2017	0
2018	1
2019	6
2020	4
Higher Risk | Unbilled
Financing Receivable, Credit Quality Indicator [Line Items]
Equipment installment plan receivables	9	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