Equipment Installment Plans - Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Allowance for credit losses
Allowance for credit losses, beginning of period	$ 65
Allowance for credit losses, end of period	67
Equipment Installment Plan Receivable
Allowance for credit losses
Allowance for credit losses, beginning of period	65	$ 50
Bad debts expense	43	42
Write-offs, net of recoveries	(41)	(34)
Allowance for credit losses, end of period	$ 67	$ 58