Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total operating revenue	$ 7,313	$ 8,052	$ 8,395
Adjustments for Non-ASC 606 Revenue	(479)	(497)	(531)
Total Revenue from Contracts with Customers	6,834	7,555	7,864
Goods and services transferred at a point in time
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	46	54	69
Services performed over time
Disaggregation of Revenue [Line Items]
Total Revenue from Contracts with Customers	6,788	7,501	7,795
IP and Data Services
Disaggregation of Revenue [Line Items]
Total operating revenue	524	595	587
Adjustments for Non-ASC 606 Revenue	0	0	0
Total Revenue from Contracts with Customers	524	595	587
Transport and Infrastructure
Disaggregation of Revenue [Line Items]
Total operating revenue	2,604	2,773	2,870
Adjustments for Non-ASC 606 Revenue	(297)	(308)	(317)
Total Revenue from Contracts with Customers	2,307	2,465	2,553
Voice and Collaboration
Disaggregation of Revenue [Line Items]
Total operating revenue	1,517	1,618	1,783
Adjustments for Non-ASC 606 Revenue	0	0	0
Total Revenue from Contracts with Customers	1,517	1,618	1,783
IT and Managed Services
Disaggregation of Revenue [Line Items]
Total operating revenue	2	4	6
Adjustments for Non-ASC 606 Revenue	0	0	0
Total Revenue from Contracts with Customers	2	4	6
Regulatory Services
Disaggregation of Revenue [Line Items]
Total operating revenue	179	189	214
Adjustments for Non-ASC 606 Revenue	(179)	(189)	(214)
Total Revenue from Contracts with Customers	0	0	0
Affiliate Services
Disaggregation of Revenue [Line Items]
Total operating revenue	2,487	2,873	2,935
Adjustments for Non-ASC 606 Revenue	(3)	0	0
Total Revenue from Contracts with Customers	$ 2,484	$ 2,873	$ 2,935