Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Allowance for credit losses
Allowance for credit losses, beginning of period	$ 78
Allowance for credit losses, end of period	72
Equipment Installment Plan Receivable
Allowance for credit losses
Allowance for credit losses, beginning of period	78	$ 84
Bad debts expense	3	25
Write-offs, net of recoveries	(9)	(19)
Allowance for credit losses, end of period	$ 72	$ 90