Affiliated and Related Party Transactions (Schedule of Affiliated Transactions - Balance Sheet) (Detail) - USD ($) $ in Millions		Sep. 30, 2021	Dec. 31, 2020
Related Party Transaction [Line Items]
Derivative assets with affiliates	[1]	$ 9	$ 0
Solar Affiliates [Member]
Related Party Transaction [Line Items]
Payable to affiliates		1	1
Nuclear Affiliates [Member]
Related Party Transaction [Line Items]
Receivable from affiliates		1
DES & DESS [Member]
Related Party Transaction [Line Items]
Payable to affiliates		20	59
Public Service Company of North Carolina, Incorporated [Member]
Related Party Transaction [Line Items]
Payable to affiliates			$ 5
Receivable from affiliates		29
DEI [Member]
Related Party Transaction [Line Items]
Payable to affiliates		$ 2

[1] Effective in the second quarter of 2021, contracts for the future purchase of certain quantities of electricity from solar affiliates no longer met the criteria for the normal purchase normal sale exception and are accounted for as derivative contracts.