Equipment Installment Plans - Narrative (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Receivables [Abstract]
Guarantee liability	$ 11	$ 11

This item represents a non-contingent liability for the fair value of an obligation to stand ready to perform over the term of a guaranty issued in the event that specified triggering events or conditions occur.

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant