Device Payment Plan Agreement and Wireless Service Receivables - Allowance for Credit Losses (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 472
Adjusted opening balance, January 1, 2020	472
Ending balance	833
Adjustments due to Topic 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Adjusted opening balance, January 1, 2020		$ 265
Device payment plan agreement
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	472
Adjusted opening balance, January 1, 2020	833
Current period provision for expected credit losses	561
Write-offs charged against the allowance	(501)
Recoveries collected	36
Ending balance	833
Device payment plan agreement | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	737
Adjusted opening balance, January 1, 2020	737
Wireless Service Plan Receivables
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	156
Adjusted opening balance, January 1, 2020	290
Current period provision for expected credit losses	342
Write-offs charged against the allowance	(256)
Recoveries collected	48
Ending balance	290
Wireless Service Plan Receivables | Cumulative Effect, Period of Adoption, Adjusted Balance
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	156
Adjusted opening balance, January 1, 2020	156
Accounting Standards Update 2016-13 | Device payment plan agreement | Adjustments due to Topic 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	265
Adjusted opening balance, January 1, 2020	265
Accounting Standards Update 2016-13 | Wireless Service Plan Receivables | Adjustments due to Topic 326
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Adjusted opening balance, January 1, 2020	$ 0