Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Allowance for credit losses
Allowance for credit losses, beginning of period	$ 84
Allowance for credit losses, end of period	82
Equipment Installment Plan Receivable
Allowance for credit losses
Allowance for credit losses, beginning of period	84	$ 77
Bad debts expense	33	38
Write-offs, net of recoveries	(35)	(35)
Allowance for credit losses, end of period	$ 82	$ 80