Device Payment Plan Agreement and Wireless Service Receivables - Schedule of Credit Quality Indicators (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Financing Receivable, Credit Quality Indicator [Line Items]
Device payment plan agreement receivables, at amortized cost	$ 16,597	$ 19,493
Device payment plan agreement
Financing Receivable, Credit Quality Indicator [Line Items]
2020	8,512
2019	7,151
2018	440
Prior to 2018	70
Device payment plan agreement receivables, at amortized cost	16,173
Device payment plan agreement | New customers
Financing Receivable, Credit Quality Indicator [Line Items]
2020	1,332
2019	1,132
2018	54
Prior to 2018	42
Device payment plan agreement receivables, at amortized cost	2,560
Device payment plan agreement | Existing customers
Financing Receivable, Credit Quality Indicator [Line Items]
2020	7,180
2019	6,019
2018	386
Prior to 2018	28
Device payment plan agreement receivables, at amortized cost	$ 13,613