Equipment Installment Plan Receivables (Gross Receivables by Credit Category) (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	$ 2,795	$ 3,558
Unbilled Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	2,585	3,291
Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	134	168
Equipment installment plan receivables, past due	76	99
Prime [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	1,164	1,707
Prime [Member] | Unbilled Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	1,084	1,593
Prime [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	54	77
Equipment installment plan receivables, past due	26	37
Subprime [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables, gross	1,631	1,851
Subprime [Member] | Unbilled Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	1,501	1,698
Subprime [Member] | Billed Revenues [Member]
Financing Receivable, Recorded Investment [Line Items]
Equipment installment plan receivables	80	91
Equipment installment plan receivables, past due	$ 50	$ 62