Rate and Other Regulatory Matters (Narrative) (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jul. 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Rate And Other Regulatory Matters [Line Items]
Interest charges			$ 63	$ 76	$ 136	$ 152
Operating expense			681	525	$ 1,489	$ 1,107
South Carolina Commission order, revenue requirement under RSA	$ 437
South Carolina Commission order, increase in natural gas rates under RSA	$ 7
Regulatory asset recovery assessment end period					2047
Monetization Of Guaranty Settlement [Member]
Rate And Other Regulatory Matters [Line Items]
Electric service customers over 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 over period					20 years
End period for recovery					2039
Asset Retirement Obligation Costs [Member]
Rate And Other Regulatory Matters [Line Items]
Recovery period of regulatory asset					106 years
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
Environmental Remediation Costs [Member]
Rate And Other Regulatory Matters [Line Items]
MPG environmental remediation					16 years
Electric Operations
Rate And Other Regulatory Matters [Line Items]
Public utilities percentage change in retail electric rates approved under BLRA		18.00%
Public utilities, approved rate increase (decrease), percentage		3.00%
Public utilities, requested rate increase (decrease), amount		$ 31
Operating expense				109
Operating expense after tax				$ 82
Reserve For Refunds To Electric Utility Customers [Member]
Rate And Other Regulatory Matters [Line Items]
Electric service customers over period					11 years
FERC
Rate And Other Regulatory Matters [Line Items]
Reserves included within revenue subject to refund			10
Interest charges			6
Interest charge, after tax			$ 4