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Equipment Installment Plan Receivables, Allowance for Credit Losses (Details) - USD ($)
$ in Millions
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Financing Receivable, Allowance for Credit Losses [Roll Forward]    
Allowance for credit losses, beginning of period $ 50  
Allowance for credit losses, end of period 57  
Equipment Installment Plan Receivable    
Financing Receivable, Allowance for Credit Losses [Roll Forward]    
Allowance for credit losses, beginning of period 50 $ 26
Bad debts expense 31 28
Write-offs, net of recoveries (24) (17)
Allowance for credit losses, end of period $ 57 $ 37