Rate and Other Regulatory Matters (Narrative) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Apr. 30, 2020	Mar. 31, 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]
Remaining useful lives of coal-fired generating units, year		2025
Demand Side Management Programs [Member]
Rate And Other Regulatory Matters [Line Items]
Recovery period of regulatory asset		5 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]
MGP environmental remediation		16 years
Subsequent Event
Rate And Other Regulatory Matters [Line Items]
Annual decrease in total fuel cost component of retail electric rates	$ 44
South Carolina Commission Order, Annual DSM Program Rate Rider Recovery Amount	40
DESC Pension Costs Rider Adjustment	$ 11