Rate and Other Regulatory Matters (Schedule of Regulatory Liabilities) (Detail) - USD ($) $ in Millions		Sep. 30, 2020	Dec. 31, 2019
Regulatory Liabilities
Regulatory liability, current		$ 291	$ 256
Regulatory liability, noncurrent		3,051	3,210
Total regulatory liabilities		3,342	3,466
Monetization Of Guaranty Settlement [Member]
Regulatory Liabilities
Regulatory liability, current	[1]	67	67
Regulatory liability, noncurrent	[1]	920	970
Income Taxes Refundable Through Future Rates [Member]
Regulatory Liabilities
Regulatory liability, current	[2]	24	16
Regulatory liability, noncurrent	[2]	917	948
Reserve For Refunds To Electric Utility Customers [Member]
Regulatory Liabilities
Regulatory liability, current	[3]	133	143
Regulatory liability, noncurrent	[3]	570	656
Cost of Fuel Over-Collections [Member]
Regulatory Liabilities
Regulatory liability, current	[4]	63	12
Other Regulatory Liability [Member]
Regulatory Liabilities
Regulatory liability, current		4	18
Regulatory liability, noncurrent		11	13
Asset Removal Cost [Member]
Regulatory Liabilities
Regulatory liability, noncurrent	[5]	563	552
Deferred Gains On Interest Rate Derivatives [Member]
Regulatory Liabilities
Regulatory liability, noncurrent	[6]	$ 70	$ 71

[1] Represents proceeds related to the monetization of the Toshiba Settlement. In accordance with the SCANA Merger Approval Order, this balance, net of amounts that may be required to satisfy liens, will be refunded to electric customers over a 20-year period ending in 2039. See Note 12 for more information.

[2] Includes (i) excess deferred income taxes arising from the remeasurement of deferred income taxes in connection with the enactment of the 2017 Tax Reform Act (certain of which are protected under normalization rules and will be amortized over the remaining lives of related property, and certain of which will be amortized to the benefit of customers over prescribed periods as instructed by regulators) and (ii) deferred income taxes arising from investment tax credits, offset by (iii) deferred income taxes that arise from utility operations that have not been included in customer rates (a portion of which relate to depreciation and are expected to be recovered over the remaining lives of the related property which may range up to 85 years). See Note 6 for more information.

[3] Reflects amounts previously collected from retail electric customers of DESC for the NND Project to be credited to customers over an estimated 11-year period (effective February 2019) in connection with the SCANA Merger Approval Order. See Note 12 for more information.

[4] Represents amounts under- or over-collected from customers pursuant to the cost of fuel and purchased gas components approved by the South Carolina Commission.

[5] Represents estimated net collections through depreciation rates of amounts to be expended for the removal of assets in the future.

[6] Represents (i) the changes in fair value and payments made or received upon settlement of certain interest rate derivatives designated as cash flow hedges and (ii) the changes in fair value and payments made or received upon settlement of certain other interest rate derivatives not so designated. The amounts recorded with respect to (i) are expected to be amortized to interest expense over the lives of the underlying debt through 2043.The amounts recorded with respect to (ii) are expected to be similarly amortized to interest expense through 2065.