Organization and Basis of Presentation (Summarized Income Statement Data) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Operating Revenues	$ 3,288	$ 3,284	$ 2,583	$ 3,022	$ 3,148	$ 3,119	$ 2,888	$ 3,514	$ 12,177	$ 12,669	$ 14,212
Operating Expenses									10,191	10,962	12,618
Net Income									1,371	1,172	1,118
Equity method investment, nonconsolidated investee or group of investees
Schedule of Equity Method Investments [Line Items]
Operating Revenues									1,227	1,210	883
Operating Expenses									847	853	622
Net Income									$ 395	$ 313	$ 365