Receivables and Expected Credit Losses - Unamortized Imputed Discount and Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 799	$ 460	$ 460
Bad debt expense	259	489
Write-offs, net of recoveries	(356)	(244)
Change in imputed discount on short-term and long-term EIP receivables	109	60
Impact on the imputed discount from sales of EIP receivables	(104)	(111)
Allowance for credit losses and imputed discount, end of period	707	745	799
Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	0	91	91
Allowance for credit losses and imputed discount, end of period			0
Accounts Receivable Allowance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	194	61	61
Bad debt expense	127	261
Write-offs, net of recoveries	(192)	(114)
Allowance for credit losses and imputed discount, end of period	129	208	194
Accounts Receivable Allowance | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 0	0	0
Allowance for credit losses and imputed discount, end of period			$ 0
EIP Receivables Allowance
Financing Receivable, Allowance for Credit Loss [Line Items]
Weighted average effective imputed interest rate (percentage)	6.10%		6.70%
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 605	399	$ 399
Bad debt expense	132	228
Write-offs, net of recoveries	(164)	(130)
Change in imputed discount on short-term and long-term EIP receivables	109	60
Impact on the imputed discount from sales of EIP receivables	(104)	(111)
Allowance for credit losses and imputed discount, end of period	578	537	605
EIP Receivables Allowance | Cumulative Effect, Period of Adoption, Adjustment
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses and imputed discount, beginning of period	$ 0	$ 91	91
Allowance for credit losses and imputed discount, end of period			$ 0