Device Payment Plan Agreement and Wireless Service Receivables - Allowance for Credit Losses (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 940
Ending balance	838
Device payment plan agreement
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	940
Current period provision for expected credit losses	349
Write-offs charged against the allowance	(482)
Recoveries collected	31
Ending balance	838
Wireless Service Plan Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	262
Current period provision for expected credit losses	126
Write-offs charged against the allowance	(296)
Recoveries collected	48
Ending balance	$ 140