Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue	$ 7,933	$ 7,773	$ 7,839
Adjustments for Non-ASC 606 Revenue	(919)	(893)	(299)
Total Revenue from Contracts with Customers	7,014	6,880	7,540
Transferred at Point in Time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue from Contracts with Customers	15	0	0
Transferred over Time
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue from Contracts with Customers	6,999	6,880	7,540
IP and Data Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue	3,587	3,655	3,728
Adjustments for Non-ASC 606 Revenue	0	0	0
Total Revenue from Contracts with Customers	3,587	3,655	3,728
Transport and Infrastructure
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue	2,615	2,544	2,591
Adjustments for Non-ASC 606 Revenue	(703)	(704)	(189)
Total Revenue from Contracts with Customers	1,912	1,840	2,402
Voice and Collaboration
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue	1,423	1,385	1,413
Adjustments for Non-ASC 606 Revenue	0	0	0
Total Revenue from Contracts with Customers	1,423	1,385	1,413
Other
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue	100	9	0
Adjustments for Non-ASC 606 Revenue	(8)	(9)	(3)
Total Revenue from Contracts with Customers	92	0	(3)
Affiliate Services
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total Revenue	208	180	107
Adjustments for Non-ASC 606 Revenue	(208)	(180)	(107)
Total Revenue from Contracts with Customers	$ 0	$ 0	$ 0