Income Taxes (Unrecognized Tax Benefits) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019		Dec. 31, 2018
Income Tax Contingency [Line Items]
Acquired unrecognized tax benefits	$ 0	$ 129	[1]	$ 0
Increases-prior period positions	18	0		10
Decreases in prior period position	$ 19	0		$ 0
SCANA
Income Tax Contingency [Line Items]
Acquired unrecognized tax benefits		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.