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Equipment Installment Plan Receivables, EIP Receivables (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Equipment installment plans [Line Items]    
Equipment installment plan receivables, gross $ 871 $ 873
Deferred interest 0 (80)
Equipment installment plan receivables, net of deferred interest 871 793
Allowance for credit losses (66) (65)
Equipment installment plan receivables, net 805 728
Accounts receivable - Customers and agents    
Equipment installment plans [Line Items]    
Equipment installment plan receivables, net 496 428
Other assets and deferred charges    
Equipment installment plans [Line Items]    
Equipment installment plan receivables, net $ 309 $ 300