Wireless Device Payment Plans - Schedule of Device Payment Plan Agreement Receivables (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement receivables, gross	$ 17,685	$ 17,770
Unamortized imputed interest	(548)	(821)
Device payment plan agreement receivables, net of unamortized imputed interest	17,137	16,949
Allowance for credit losses	(662)	(848)	$ (727)	$ (688)
Device payment plan agreement receivables, net	16,475	16,101
Account receivable, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement receivables, net	11,708	11,064
Other assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement receivables, net	$ 4,767	$ 5,037