Acquisitions and Dispositions (Acquisition of SCANA) (Narrative) (Detail) - USD ($) shares in Millions, $ in Millions	1 Months Ended	3 Months Ended
	Jan. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Business Acquisition And Dispositions [Line Items]
Stock issued during period for acquisition, value			$ 6,818
Regulatory Liabilities		$ 11,230		$ 11,498
After tax charge in statements of income		(270)	(680)
Impairment of assets and other charges		$ 768	$ 835
SCANA
Business Acquisition And Dispositions [Line Items]
Stock issued during period for acquisition, shares	95.6
Stock issued during period for acquisition, value	$ 6,800
Common stock agreed to be issued, percentage	0.669%
Total outstanding debt	$ 6,900
Period to provide refund to customer			11 years
Regulatory Liabilities			$ 1,000
Reduction in operating revenue			(1,000)
After tax charge in statements of income			(756)
Impairment of assets and other charges			178
Impairment of assets and other charges, after tax			133
SCANA | NND Project
Business Acquisition And Dispositions [Line Items]
Property, plant and equipment	105
Impairment of assets and other charges			105
Impairment of assets and other charges, after tax			79
Income tax regulatory assets	$ 264
SCANA | NND Project | Income tax expense (benefit)
Business Acquisition And Dispositions [Line Items]
Impairment of assets and other charges			$ 198