Employee Benefit Plans (Fair Values of Pension and Post Retirement Plan Assets by Asset Category) (Detail) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[1]	$ 10,831	$ 9,654
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[2]	2,091	1,881
Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		4,214	4,271
Level 1 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1,061	930
Level 2 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		2,567	1,977
Level 2 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		127	115
Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		6,781	6,248
Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1,188	1,045
NAV | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	4,050	3,406
Cash equivalents and other | Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		20	22
Cash equivalents and other | Level 1 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value			2
Cash equivalents and other | Level 2 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1	1
Cash equivalents and other | Level 2 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		2
Cash equivalents and other | Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		21	23
Cash equivalents and other | Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		2	2
US Equity Securities | Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[4]	2,405	2,284
US Equity Securities | Level 1 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		817	719
US Equity Securities | Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[4]	2,405	2,284
US Equity Securities | Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		817	719
Foreign Equity Securities | Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1,727	1,634
Foreign Equity Securities | Level 1 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		240	206
Foreign Equity Securities | Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1,727	1,634
Foreign Equity Securities | Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		240	206
Insurance Contracts | Level 2 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		409	360
Insurance Contracts | Level 2 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		23	21
Insurance Contracts | Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		409	360
Insurance Contracts | Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		23	21
Corporate debt instruments | Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		32	273
Corporate debt instruments | Level 1 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		2	1
Corporate debt instruments | Level 2 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1,385	859
Corporate debt instruments | Level 2 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		60	50
Corporate debt instruments | Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		1,417	1,132
Corporate debt instruments | Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		62	51
Government securities | Level 1 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		30	58
Government securities | Level 1 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		2	2
Government securities | Level 2 | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		772	757
Government securities | Level 2 | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		42	44
Government securities | Total Fair Value | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		802	815
Government securities | Total Fair Value | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value		44	46
Common/collective trust funds | NAV | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	2,905	2,355
Common/collective trust funds | NAV | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	765	717
Real Estate Funds | NAV | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	108	91
Real Estate Funds | NAV | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	10	8
Private Equity Funds | NAV | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	856	787
Private Equity Funds | NAV | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	117	100
Fixed Income Funds | NAV | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	168	159
Fixed Income Funds | NAV | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	10	10
Hedge Funds | NAV | Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	13	14
Hedge Funds | NAV | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	1	1
Investments Measured At Fair Value Including Excluding Pending Purchases Sales And Accrued Income | NAV | Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Total recorded at fair value	[3]	$ 903	$ 836

[1] Excludes net assets related to pending sales of securities and advanced subscriptions of $198 million, net accrued income of $20 million, and includes net assets related to pending purchases of securities of $71 million at December 31, 2020. Excludes net assets related to pending sales of securities of $52 million, net accrued income of $24 million, and includes net assets related to pending purchases of securities of $99 million at December 31, 2019.

[2] Excludes net assets related to pending sales of securities of $10 million, net accrued income of $2 million, and includes net assets related to pending purchases of securities of $2 million at December 31, 2020. Excludes net assets related to pending sales of securities of $2 million, net accrued income of $2 million, and includes net assets related to pending purchases of securities of $5 million at December 31, 2019.

[3] These investments that are measured at fair value using the NAV per share (or its equivalent) as a practical expedient are not required to be categorized in the fair value hierarchy.

[4] Includes $365 million and $508 million of Dominion Energy common stock at December 31, 2020 and 2019, respectively.