Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Allowance for credit losses
Allowance for credit losses, beginning of period	$ 77
Allowance for credit losses, end of period	82
Equipment Installment Plan Receivable
Allowance for credit losses
Allowance for credit losses, beginning of period	77	$ 65
Bad debts expense	60	49
Write-offs, net of recoveries	(55)	(41)
Allowance for credit losses, end of period	$ 82	$ 73