Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Allowance for credit losses
Allowance for credit losses, beginning of period	$ 84
Allowance for credit losses, end of period	75
Equipment Installment Plan Receivable
Allowance for credit losses
Allowance for credit losses, beginning of period	84	$ 77
Bad debts expense	34	60
Write-offs, net of recoveries	(43)	(55)
Allowance for credit losses, end of period	$ 75	$ 82