Fair Value Measurements, Including Derivatives (Schedule of Liabilities Measured at Fair Value on Recurring Basis) (Narrative) (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impairment of assets and other charges		$ 105	$ (14)	$ 576	$ 1,376
Impairment of assets and other charges, after-tax		$ 79
Nonutility Property
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Impairment of assets and other charges	$ 12
Impairment of assets and other charges, after-tax	9
Estimated fair value of property	$ 6