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Equipment Installment Plan Receivables, EIP Receivables (Details) - USD ($)
$ in Millions
Sep. 30, 2017
Dec. 31, 2016
Equipment installment plans [Line Items]    
Equipment installment plan receivables, gross $ 776 $ 628
Deferred interest (69) (53)
Equipment installment plan receivables, net of deferred interest 707 575
Allowance for credit losses (58) (50)
Equipment installment plan receivables, net 649 525
Accounts receivable - Customers and agents    
Equipment installment plans [Line Items]    
Equipment installment plan receivables, net 387 345
Other assets and deferred charges    
Equipment installment plans [Line Items]    
Equipment installment plan receivables, net $ 262 $ 180