Background and Summary of Significant Accounting Policies (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) reporting_unit segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)
Description of Business
Past due threshold, days outstanding	30 days
Finite-lived intangible assets, useful life	9 years
Number of reporting units | reporting_unit	1
Number of operating segments | segment	1
Revenues	$ 7,933	$ 7,773	$ 7,839
Cost of services	3,486	3,387	3,556
Income tax expense (benefit)	221	255	196
Operating lease assets	1,091	1,060			$ 1,300
Operating lease liabilities	1,144				$ 1,400
Member's equity
Description of Business
Partners' capital	13,139	13,724	18,048	$ 19,254
Accumulated other comprehensive income
Description of Business
Partners' capital	(234)	(179)	(171)	18
Accumulated other comprehensive income | Accounting Standards Update 2018-02
Description of Business
Partners' capital				6
Cumulative Effect, Period of Adoption, Adjustment | Member's equity
Description of Business
Partners' capital		(3)	(39)
Income tax expense (benefit)		(2)
Cumulative Effect, Period of Adoption, Adjustment | Member's equity | Accounting Standards Update 2014-09
Description of Business
Partners' capital				9
Income tax expense (benefit)				3
Cumulative Effect, Period of Adoption, Adjustment | Member's equity | Accounting Standards Update 2018-02
Description of Business
Partners' capital				$ (6)
Change in Accounting Principle, Universal Service Fund
Description of Business
Revenues	(398)	(412)	(381)
Cost of services	$ (398)	$ (412)	$ (381)
Customer relationships
Description of Business
Finite-lived intangible assets, useful life	9 years
Capitalized software
Description of Business
Finite-lived intangible assets, useful life	7 years
Other
Description of Business
Finite-lived intangible assets, useful life	5 years
Minimum
Description of Business
Contract term	1 year
Period company may receive up front payments for services to be provided in the future (in years)	10 years
Minimum | Customer relationships
Description of Business
Finite-lived intangible assets, useful life	7 years
Maximum
Description of Business
Contract term	5 years
Period company may receive up front payments for services to be provided in the future (in years)	20 years
Maximum | Customer relationships
Description of Business
Finite-lived intangible assets, useful life	14 years
Weighted Average | Consumer Customers
Description of Business
Length of customer life	30 months