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Equipment Installment Plans - EIP Receivables (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Equipment installment plan receivables, gross $ 919 $ 873
Deferred interest 0 (80)
Equipment installment plan receivables, net of deferred interest 919 793
Allowance for credit losses (67) (65)
Equipment installment plan receivables, net 852 728
Accounts receivable — Customers and agents (Current portion)    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Equipment installment plan receivables, net 537 428
Other assets and deferred charges (Non-current portion)    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Equipment installment plan receivables, net $ 315 $ 300