Device Payment Plan Agreement and Wireless Service Receivables - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jan. 01, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables transferred to ABS Entities	$ 12,300		$ 14,300
Device payment plan agreement, trade-in liability	$ 5,321	$ 5,307	5,307	$ 5,082	$ 4,943
Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Required down payment percentage	0.00%
Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Required down payment percentage	100.00%
Product Trade-in
Accounts, Notes, Loans and Financing Receivable [Line Items]
Device payment plan agreement, trade-in liability			$ 103
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum threshold period for new customers	210 days
Minimum threshold period for existing customers	210 days
Business
Accounts, Notes, Loans and Financing Receivable [Line Items]
Maximum threshold period for new customers	12 months
Minimum threshold period for existing customers	12 months
Device payment plan agreement
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables originating in 2020	$ 8,512
Receivables originating in 2019	$ 7,151
Device payment plan agreement | Consumer | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer tenure period	45 days
Device payment plan agreement | Consumer | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer tenure period	45 days
Device payment plan agreement | Business | Minimum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer tenure period	12 months
Device payment plan agreement | Business | Maximum
Accounts, Notes, Loans and Financing Receivable [Line Items]
Customer tenure period	12 months
Wireless Service Plan Receivables
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables originating in 2020	$ 5,500
Number of days past due	34 days