Receivables and Expected Credit Losses - EIP Receivables (Details) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($) segment class	Dec. 31, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Portfolio segments | segment	2
Customer classes | class	2
EIP receivables, gross	$ 6,945	$ 6,213
Unamortized imputed discount	(342)	(325)
EIP receivables, net of unamortized imputed discount	6,603	5,888
Allowance for credit losses	(255)	(280)
Equipment installment plan receivables, net	6,348	5,608
Equipment installment plan receivables, net of allowance for credit losses and imputed discount
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equipment installment plan receivables, net	4,064	3,577
Equipment installment plan receivables due after one year, net of allowance for credit losses and imputed discount
Accounts, Notes, Loans and Financing Receivable [Line Items]
Equipment installment plan receivables, net	$ 2,284	$ 2,031