Segment Information - Segment Results and Operating Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Operating revenues by products and services
Revenues	$ 4,887	$ 5,167	$ 14,840	$ 15,587
Cost of services and products	2,151	2,236	6,402	6,703
Selling, general and administrative	654	850	2,172	2,598
Less: share-based compensation	(27)	(31)	(89)	(120)
Total expense	2,778	3,055	8,485	9,181
Total adjusted EBITDA	2,109	2,112	6,355	6,406
Operating Segments
Operating revenues by products and services
Revenues	4,887	5,167	14,840	15,587
Cost of services and products	904	967	2,729	2,890
Selling, general and administrative	421	447	1,308	1,407
Less: share-based compensation	0	0	0	0
Total expense	1,325	1,414	4,037	4,297
Total adjusted EBITDA	3,562	3,753	10,803	11,290
Operating Segments | Business
Operating revenues by products and services
Revenues	3,508	3,698	10,625	11,152
Cost of services and products	868	912	2,611	2,738
Selling, general and administrative	296	299	900	975
Less: share-based compensation	0	0	0	0
Total expense	1,164	1,211	3,511	3,713
Total adjusted EBITDA	2,344	2,487	7,114	7,439
Operating Segments | Mass Markets
Operating revenues by products and services
Revenues	1,379	1,469	4,215	4,435
Cost of services and products	36	55	118	152
Selling, general and administrative	125	148	408	432
Less: share-based compensation	0	0	0	0
Total expense	161	203	526	584
Total adjusted EBITDA	1,218	1,266	3,689	3,851
Operations and Other
Operating revenues by products and services
Revenues	0	0	0	0
Cost of services and products	1,247	1,269	3,673	3,813
Selling, general and administrative	233	403	864	1,191
Less: share-based compensation	(27)	(31)	(89)	(120)
Total expense	1,453	1,641	4,448	4,884
Total adjusted EBITDA	$ (1,453)	$ (1,641)	$ (4,448)	$ (4,884)