Income Taxes (Unrecognized Tax Benefits) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019		Dec. 31, 2018	Dec. 31, 2017
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition	$ 129	[1]	$ 0	$ 0
Increases-prior period positions	0		10	1
Decreases in prior period position	0		$ 0	$ 9
SCANA
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Increase Resulting from Acquisition	106
Increases-prior period positions	76
Decreases in prior period position	$ 53

[1] Acquired unrecognized tax benefits reflect $106 million plus increases in prior period positions of $76 million and decreases in prior period positions of $53 million that were recorded through purchase accounting.