Summary of Significant Accounting Policies (COVID 19) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Noncontrolling Interest [Line Items]
Operation, maintenance and other	$ 5,788	$ 6,066	$ 6,463
Incremental COVID-19 costs
Noncontrolling Interest [Line Items]
Operation, maintenance and other	112
Waived late payment fees	64
Deferred costs	$ 76