Wireless Licenses, Goodwill and Other Intangible Assets - Composition of Other Intangible Assets, Net (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 28,278	$ 26,393
Accumulated Amortization	(18,679)	(16,895)
Net Amount	9,599	9,498
Customer Lists
Finite-Lived Intangible Assets [Line Items]
Gross Amount	4,048	3,896
Accumulated Amortization	(1,854)	(1,511)
Net Amount	$ 2,194	2,385
Customer Lists | Minimum
Finite-Lived Intangible Assets [Line Items]
Other intangible asset, useful life	8 years
Customer Lists | Maximum
Finite-Lived Intangible Assets [Line Items]
Other intangible asset, useful life	13 years
Non-Network Internal-Use Software
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 22,229	20,530
Accumulated Amortization	(15,769)	(14,418)
Net Amount	$ 6,460	6,112
Non-Network Internal-Use Software | Minimum
Finite-Lived Intangible Assets [Line Items]
Other intangible asset, useful life	5 years
Non-Network Internal-Use Software | Maximum
Finite-Lived Intangible Assets [Line Items]
Other intangible asset, useful life	7 years
Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 2,001	1,967
Accumulated Amortization	(1,056)	(966)
Net Amount	$ 945	$ 1,001
Other | Minimum
Finite-Lived Intangible Assets [Line Items]
Other intangible asset, useful life	2 years
Other | Maximum
Finite-Lived Intangible Assets [Line Items]
Other intangible asset, useful life	25 years