Segment Information - Segment Results and Operating Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Operating revenues by products and services
Revenues	$ 4,924	$ 5,192	$ 9,953	$ 10,420
Cost of services and products	2,115	2,232	4,251	4,467
Selling, general and administrative	762	895	1,518	1,748
Less: share-based compensation	(42)	(20)	(62)	(89)
Total expense	2,835	3,107	5,707	6,126
Total adjusted EBITDA	2,089	2,085	4,246	4,294
Operating Segments
Operating revenues by products and services
Revenues	4,924	5,192	9,953	10,420
Cost of services and products	901	967	1,825	1,923
Selling, general and administrative	448	478	887	960
Less: share-based compensation	0	0	0	0
Total expense	1,349	1,445	2,712	2,883
Total adjusted EBITDA	3,575	3,747	7,241	7,537
Operating Segments | Business
Operating revenues by products and services
Revenues	3,522	3,717	7,117	7,454
Cost of services and products	862	919	1,743	1,826
Selling, general and administrative	298	334	604	676
Less: share-based compensation	0	0	0	0
Total expense	1,160	1,253	2,347	2,502
Total adjusted EBITDA	2,362	2,464	4,770	4,952
Operating Segments | Mass Markets
Operating revenues by products and services
Revenues	1,402	1,475	2,836	2,966
Cost of services and products	39	48	82	97
Selling, general and administrative	150	144	283	284
Less: share-based compensation	0	0	0	0
Total expense	189	192	365	381
Total adjusted EBITDA	1,213	1,283	2,471	2,585
Operations and Other
Operating revenues by products and services
Revenues	0	0	0	0
Cost of services and products	1,214	1,265	2,426	2,544
Selling, general and administrative	314	417	631	788
Less: share-based compensation	(42)	(20)	(62)	(89)
Total expense	1,486	1,662	2,995	3,243
Total adjusted EBITDA	$ (1,486)	$ (1,662)	$ (2,995)	$ (3,243)