Rate and Other Regulatory Matters (Narrative) (Detail) - USD ($) $ in Millions	1 Months Ended				9 Months Ended
	Oct. 31, 2020	Aug. 31, 2020	Jun. 30, 2020	Apr. 30, 2020	Sep. 30, 2020
Rate And Other Regulatory Matters [Line Items]
Regulatory asset recovery assessment end period					2047
Reserve For Refunds To Electric Utility Customers [Member]
Rate And Other Regulatory Matters [Line Items]
Electric service customers recovery period					11 years
Monetization Of Guaranty Settlement [Member]
Rate And Other Regulatory Matters [Line Items]
Electric service customers recovery period					20 years
End period for recovery					2039
Income Taxes Refundable Through Future Rates [Member]
Rate And Other Regulatory Matters [Line Items]
Remaining lives of related property period					85 years
Deferred Losses or Gains On Interest Rate Derivatives [Member]
Rate And Other Regulatory Matters [Line Items]
Changes in fair value and payments of interest rate derivatives designated as cash flow hedge, amortized to interest expense, year					2043
Changes in fair value and payments of interest rate derivatives not designed, amortized to interest, year					2065
NND Project Costs [Member]
Rate And Other Regulatory Matters [Line Items]
Electric service customers recovery period					20 years
End period for recovery					2039
Deferred Employee Benefit Plan Costs [Member]
Rate And Other Regulatory Matters [Line Items]
Regulatory asset recovery assessment end period					2044
Average service period expected to recover other deferred benefit costs					11 years
Other Unrecovered Plant [Member]
Rate And Other Regulatory Matters [Line Items]
Amortization of carrying value of coal-fired generating unit					2025
Demand Side Management Programs [Member]
Rate And Other Regulatory Matters [Line Items]
Recovery period of regulatory asset					3 years
Asset Retirement Obligation Costs [Member]
Rate And Other Regulatory Matters [Line Items]
Recovery period of regulatory asset					105 years
Cost of Reacquired Debt [Member]
Rate And Other Regulatory Matters [Line Items]
Recovery period of regulatory asset					26 years
Environmental Remediation Costs [Member]
Rate And Other Regulatory Matters [Line Items]
Recovery period of regulatory asset					16 years
Fuel Component [Member]
Rate And Other Regulatory Matters [Line Items]
Annual increase (decrease) in total fuel cost component of retail electric rates				$ (44)
DSM Programs [Member]
Rate And Other Regulatory Matters [Line Items]
South Carolina Commission Order, Annual DSM Program Rate Rider Recovery Amount				40
Pension Costs [Member]
Rate And Other Regulatory Matters [Line Items]
South Carolina Commission Order, Annual DSM Program Rate Rider Recovery Amount				$ 11
Natural Gas Rates [Member]
Rate And Other Regulatory Matters [Line Items]
Annual increase (decrease) in total fuel cost component of retail electric rates			$ 9
South Carolina Commission order, revenue requirement under Natural Gas Rate Stabilization Act			$ 409
Natural Gas Rates [Member] | Subsequent Event
Rate And Other Regulatory Matters [Line Items]
Percentage of current authorized earned ROE	9.90%
Annual increase (decrease) in total fuel cost component of retail electric rates	$ 6
South Carolina Commission order, revenue requirement under Natural Gas Rate Stabilization Act	$ 406
Percentage of prior authorized earned ROE	10.25%
South Carolina Electric Base Rate Case
Rate And Other Regulatory Matters [Line Items]
Amount of Increase in proposed non fuel base rate		$ 178
Increase in proposed non fuel base rate		7.75%
Percentage of ROE based on fully adjusted test period		5.90%
Percentage of current authorized earned ROE		10.25%