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Equipment Installment Plans - Narrative (Details) - USD ($) $ in Millions | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2016 | Dec. 31, 2018 | Dec. 31, 2017
Receivables [Abstract] |  |  |
Guarantee liability |  | $ 11 | $ 15
Equipment installment plans out-of-period adjustment | Equipment sales revenues |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Out-of-period adjustment | $ 2 |  |
Equipment installment plans out-of-period adjustment | Selling, general and administrative expense |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Out-of-period adjustment | (2) |  |
Equipment installment plans out-of-period adjustment | Income before income taxes |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Out-of-period adjustment | $ 4 |  |

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