Equipment Installment Plan Receivables, Allowance for Credit Losses (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses, beginning of period	$ 50
Allowance for credit losses, end of period	58
Equipment Installment Plan Receivable
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Allowance for credit losses, beginning of period	50	$ 26
Bad debts expense	42	46
Write-offs, net of recoveries	(34)	(28)
Allowance for credit losses, end of period	$ 58	$ 44