Equipment Installment Plan Receivables Unamortized Imputed Discount and Allowance for Credit Losses (Details) - Equipment Installment Plan Receivable [Member] - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]
Imputed discount and allowance for credit losses, beginning of period	$ 316	$ 333	$ 333
Bad debt expense	215	185
Write-offs, net of recoveries	(205)	(201)
Change in imputed discount on short-term and long-term EIP receivables	163	103
Impacts from sales of EIP receivables	(126)	(133)
Imputed discount and allowance for credit losses, end of period	$ 363	$ 287	$ 316
Weighted average effective imputed interest rate	9.70%		9.00%