Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for credit losses
Allowance for credit losses, beginning of period	$ 84
Allowance for credit losses, end of period	90
Equipment Installment Plan Receivable
Allowance for credit losses
Allowance for credit losses, beginning of period	84	$ 77
Bad debts expense	25	18
Write-offs, net of recoveries	(19)	(18)
Allowance for credit losses, end of period	$ 90	$ 77