Background (Details) $ in Millions		3 Months Ended			9 Months Ended		10 Months Ended	12 Months Ended
	Jan. 01, 2020	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment	Sep. 30, 2019 USD ($)	Nov. 05, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018	Oct. 15, 2020
Accounting Policies [Abstract]
Number of reportable segments | segment					1
New Accounting Pronouncement, Early Adoption [Line Items]
Cost of services and products reduction			$ (857)	$ (848)	$ (2,623)	$ (2,550)
Revenue reduction			(1,971)	(1,952)	(5,935)	(5,828)
Rental income			$ 197	$ 52	$ 565	$ 153
Percent of operating revenue			10.00%	3.00%	10.00%	3.00%
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member							us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Member's equity			$ (12,738)	$ (13,653)	$ (12,738)	$ (13,653)		$ (13,545)
Subsequent Event
New Accounting Pronouncement, Early Adoption [Line Items]
Dividends declared							$ 1,200
Subsequent Event | Revolving Credit Facility | Line of Credit
New Accounting Pronouncement, Early Adoption [Line Items]
Stated interest rate										4.25%
Forecast
New Accounting Pronouncement, Early Adoption [Line Items]
Dividends declared		$ 125
Cumulative Effect, Period of Adoption, Adjustment | Accounting Standards Update 2016-13
New Accounting Pronouncement, Early Adoption [Line Items]
Member's equity								$ 3
Change in Accounting Principle, Universal Service Fund
New Accounting Pronouncement, Early Adoption [Line Items]
Cost of services and products reduction			(113)	(112)	(301)	(296)
Revenue reduction			$ (113)	$ (112)	$ (301)	$ (296)