Wireless Device Payment Plans - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables transferred to ABS Entities		$ 12,200		$ 10,700
Device payment plan agreement, trade-in liability		$ 4,693		$ 4,452
Number of customer tenure days		210 days
Credit losses on receivables sold			$ 100
Cash flows used in investing activities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred purchase price receivable collected	$ 600	$ 200	$ 1,100
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		$ 100