Wireless Licenses, Goodwill and Other Intangible Assets - Composition of Other Intangible Assets, Net (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 25,008	$ 24,105
Accumulated Amortization	(15,277)	(13,858)
Net Amount	9,731	10,247
Customer Lists
Finite-Lived Intangible Assets [Line Items]
Gross Amount	3,623	3,621
Accumulated Amortization	(945)	(691)
Net Amount	$ 2,678	2,930
Customer Lists | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets	8 years
Customer Lists | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets	13 years
Non-Network Internal-Use Software
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 18,901	18,010
Accumulated Amortization	(13,371)	(12,374)
Net Amount	$ 5,530	5,636
Non-Network Internal-Use Software | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets	3 years
Non-Network Internal-Use Software | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets	7 years
Other
Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 2,484	2,474
Accumulated Amortization	(961)	(793)
Net Amount	$ 1,523	$ 1,681
Other | Minimum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets	2 years
Other | Maximum
Finite-Lived Intangible Assets [Line Items]
Useful life for finite-lived intangible assets	25 years