Wireless Device Payment Plans - Additional Information (Detail) - USD ($) $ in Billions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Receivables transferred to ABS Entities		$ 12.4
Number of customer tenure days		210 days
Cash flows used in investing activities
Accounts, Notes, Loans and Financing Receivable [Line Items]
Deferred purchase price receivable collected	$ 0.3	$ 0.2	$ 0.6
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%