Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2021	Jun. 30, 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	13	33
Write-offs, net of recoveries	(19)	(35)
Allowance for credit losses, end of period	$ 72	$ 82