Device Payment Plan Agreement and Wireless Service Receivables - Schedule of Device Payment Plan Agreement Receivables (Detail) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement receivables, gross	$ 18,991	$ 17,959
Unamortized imputed interest	(365)	(453)
Device payment plan agreement receivables, at amortized cost	18,626	17,506
Allowance	(838)	(940)
Device payment plan agreement receivables, net	17,788	16,566
Account receivable, net
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement receivables, net	11,797	11,601
Other assets
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement receivables, net	$ 5,991	$ 4,965