Wireless Licenses, Goodwill and Other Intangible Assets - Composition of Other Intangible Assets, Net (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 27,477	$ 26,393
Accumulated Amortization	(18,064)	(16,895)
Net Amount	9,413	9,498
Customer lists
Finite-Lived Intangible Assets [Line Items]
Gross Amount	4,021	3,896
Accumulated Amortization	(1,961)	(1,511)
Net Amount	$ 2,060	2,385
Customer lists | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets, years	8 years
Customer lists | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets, years	13 years
Non-Network Internal-Use Software
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 21,685	20,530
Accumulated Amortization	(15,104)	(14,418)
Net Amount	$ 6,581	6,112
Non-Network Internal-Use Software | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets, years	5 years
Non-Network Internal-Use Software | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets, years	7 years
Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 1,771	1,967
Accumulated Amortization	(999)	(966)
Net Amount	$ 772	$ 1,001
Other | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets, years	2 years
Other | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets, years	25 years