Acquisitions and Divestitures - Acquisitions (Details) customer in Thousands, $ in Millions	1 Months Ended								3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Feb. 28, 2021 USD ($) license	Oct. 31, 2020 customer county	Sep. 30, 2020 USD ($) license shares	May 31, 2020 USD ($)	Mar. 31, 2020 USD ($) license	Dec. 31, 2019 USD ($)	Sep. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Acquisition [Line Items]
Payments for deposits												$ 47,027	$ 3,757
Loss on disposition of licenses, pre-tax									$ 0		$ 0	223	1,195
Goodwill	$ 24,887								24,887			$ 24,887		$ 24,773
Spectrum licenses | Spectrum Licenses, C-Band
Business Acquisition [Line Items]
Number of licenses acquired | license			3,511
Payments for deposits	1,300		$ 45,500							$ 44,600
Estimated clearing and incentive costs			$ 7,700
Average remaining renewal period of wireless license portfolio (in years)												15 years
Spectrum Licenses, 39 GHz
Business Acquisition [Line Items]
Payments for deposits							$ 1,600	$ 101
Loss on disposition of licenses, pre-tax														1,200
Loss on disposition of licenses														$ 914
Spectrum licenses
Business Acquisition [Line Items]
Number of licenses acquired | license							4,940
Business acquisition consideration							$ 3,400
Portion of price to be settled with 39 GHz licenses							$ 1,800
Loss on disposition of licenses, pre-tax													1,200
Loss on disposition of licenses													914
Priority Access Licenses
Business Acquisition [Line Items]
Number of licenses acquired | license					557
Payments for deposits					$ 1,900
Various Other Wireless Licenses | Spectrum licenses
Business Acquisition [Line Items]
Payments for deposits												$ 95
Loss on disposition of licenses, pre-tax												223
Loss on disposition of licenses												167
Blue Jeans Network
Business Acquisition [Line Items]
Business acquisition consideration						$ 397
TracFone Wireless, Inc.
Business Acquisition [Line Items]
Cash payment to acquire business					3,100
Common stock value consideration to acquire business					3,100
Future cash consideration related to achievement of certain performance measures					$ 650						$ 650		$ 650
TracFone Wireless, Inc. | Minimum
Business Acquisition [Line Items]
Number of shares issuable (in shares) | shares					47,124,445
TracFone Wireless, Inc. | Maximum
Business Acquisition [Line Items]
Number of shares issuable (in shares) | shares					57,596,544
Bluegrass Cellular, Certain Assets
Business Acquisition [Line Items]
Business acquisition consideration		$ 412
Number of customers | customer				210
Number of counties customers are serviced | county				34
Revenue from the date of acquisition												88
Plant, property and equipment	141								141			141
Intangible assets other than goodwill	135								135			135
Goodwill	$ 92								$ 92			$ 92