Background (Details) $ in Millions		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 01, 2020 USD ($)	Sep. 30, 2020 USD ($) state	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) segment state	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of states in which entity operates | state		14		14
Number of reportable segments | segment				1
Operating revenues		$ 1,801	$ 2,011	$ 5,518	$ 6,069
Cost of services and products (exclusive of depreciation and amortization)		528	602	1,491	1,749
Lease income		$ 77	79	$ 234	242
Percent of operating revenue		4.00%		4.00%
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member					us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Cumulative adjustment		$ 10,077	10,231	$ 10,077	10,231	$ 10,117
RETAINED EARNINGS (ACCUMULATED DEFICIT)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment		27	181	27	181	67	$ (182)	$ 73	$ 104
RETAINED EARNINGS (ACCUMULATED DEFICIT) | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative adjustment	$ 3					$ 3	$ 22
Change in Accounting Principle, Universal Service Fund
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating revenues		28	28	75	76
Cost of services and products (exclusive of depreciation and amortization)		$ 28	$ 28	$ 75	$ 76