Wireless Licenses, Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020
Indefinite-lived Intangible Assets [Line Items]
Loss on disposition of licenses, pre-tax	$ 0	$ 0	$ 223	$ 1,195
Wireless licenses	145,767		145,767		$ 96,097
Wireless Licenses
Indefinite-lived Intangible Assets [Line Items]
Wireless licenses under development	$ 54,800	$ 6,900	54,800	6,900
Interest costs capitalized			$ 1,100	$ 171
Assets held-for-sale							$ 2,800
Average remaining renewal period of wireless license portfolio (in years)			10 years
Spectrum Licenses, 39 GHz
Indefinite-lived Intangible Assets [Line Items]
Loss on disposition of licenses, pre-tax					1,200
Loss on disposition of licenses					$ 914
Wireless licenses						$ 3,400
Disposal Group, Held-for-sale, Not Discontinued Operations | Spectrum licenses
Indefinite-lived Intangible Assets [Line Items]
Other intangible assets, net							$ 1,600