Commitments and Contingencies (Narrative) (Detail) shares in Millions		1 Months Ended													3 Months Ended	6 Months Ended		9 Months Ended
	Oct. 31, 2014 MGD Facility	Aug. 31, 2021 shares	Jul. 31, 2021 USD ($) shares	Jun. 30, 2021 USD ($) shares	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Aug. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jul. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)	Aug. 31, 2017 USD ($)	Aug. 31, 2016 T	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($) Product	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)
Loss Contingencies [Line Items]
Electric generating stations with water withdrawals under CWA | MGD	125
Electric generating stations with water withdrawals with heightened entrainment analysis under CWA | MGD	2
Number of DESC facilities subject to final regulations | Facility	5
Number of DESC hydroelectric facilities subject to regulations | Facility	5
Number of manufacturing gas plant decommissioned sites that contain residues of byproduct chemicals | Product																4
Estimated environmental remediation activities at manufacturing gas plant sites																$ 9,000,000
Estimated increase in remediation costs for Congaree River site																11,000,000
Environmental remediation costs recognized in regulatory assets				$ 22,000,000											$ 22,000,000	22,000,000
Reserves for litigation and regulatory proceedings				278,000,000		$ 208,000,000									278,000,000	278,000,000
Insurance receivables				23,000,000		8,000,000									23,000,000	23,000,000
Impairment of assets and other charges																319,000,000	$ 2,000,000
Settlement payable by all																			$ 520,000,000
Settlement payable by company			$ 99,000,000										$ 100,000,000						$ 320,000,000
Proportionate ownership share in project												100.00%	100.00%
Proposed assessment amount from SCDOR audit												$ 410,000,000
Litigation settlement expense awarded						165,000,000
Contesting amount for filed liens in Fairfield country																285,000,000
Reduction of liens filed																$ 60,000,000
Percentage share of reduction of liens filed																55.00%
Nuclear Insurance
Loss Contingencies [Line Items]
Maximum liability protection per nuclear incident amount				$ 13,500,000,000	$ 13,700,000,000
Common Stock | Dominion Energy
Loss Contingencies [Line Items]
Common stock issued to satisfy the settlement																		$ 322,000,000
Shares issued on settlement | shares		0.6	1.4	43.0
DESC Ratepayer Case
Loss Contingencies [Line Items]
Escrow amount											$ 2,000,000,000.0
Credit in future electric rate relief for ratepayer case											2,000,000,000.0
Cash payment related to Ratepayer Case										$ 117,000,000	115,000,000
Property with net value transferred								$ 22,000,000
Property with net value transferred, cash contribution							$ 38,500,000
Conveyance of property with net recorded value							$ 3,000,000
Minimum [Member] | DESC Ratepayer Case
Loss Contingencies [Line Items]
Estimated aggregate fair value of certain real estate											60,000,000
Maximum [Member] | Nuclear Insurance
Loss Contingencies [Line Items]
Maximum liability protection per nuclear incident amount				$ 13,700,000,000	$ 13,800,000,000
Maximum [Member] | DESC Ratepayer Case
Loss Contingencies [Line Items]
Estimated aggregate fair value of certain real estate											$ 85,000,000
Dominion Energy South Carolina, Inc.
Loss Contingencies [Line Items]
Reduction of liens filed																$ 33,000,000
Dominion Energy South Carolina, Inc. | Wrongful Death Suit of Estate of Jose Larios
Loss Contingencies [Line Items]
Litigation settlement expense awarded									$ 19,000,000
SCANA
Loss Contingencies [Line Items]
Payment of civil monetary penalty						25,000,000
SCANA and DESC
Loss Contingencies [Line Items]
Settlement payable by company				$ 0											0	0
Payment of disgorgement and prejudgment interest						$ 112,500,000
Impairment Of Assets And Other Charges | Dominion Energy South Carolina, Inc.
Loss Contingencies [Line Items]
Impairment of assets and other charges															10,000,000	70,000,000
Impairment of assets and other charges, after-tax															$ 8,000,000	$ 53,000,000
Carbon Regulations [Member]
Loss Contingencies [Line Items]
Significant emission rate per year CO2 equivalent | T														75,000