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Equipment Installment Plan Receivables - Allowance for Credit Losses (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Allowance for credit losses    
Allowance for credit losses, beginning of year $ 65  
Allowance for credit losses, end of year 70 $ 65
Equipment Installment Plan Receivable    
Allowance for credit losses    
Allowance for credit losses, beginning of year 65 50
Bad debts expense 64 62
Write-offs, net of recoveries (59) (47)
Allowance for credit losses, end of year $ 70 $ 65