Investments (Equity and Fixed Income Securities, Insurance Contracts and Cash Equivalents in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Fixed income securities Fair Value		$ 1,988
Amortized Cost, Total		3,846		$ 3,691
Total Unrealized Gains		3,081		2,523
Total Unrealized Losses	[1]	(27)		(22)
Fair Value, Total		6,900		6,192
Net assets related to pending purchases of securities		49		1
Fair value of securities in an unrealized loss position		293		298
Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Fair Value		915
Amortized Cost, Total		1,789		1,721
Total Unrealized Gains		1,429		1,173
Total Unrealized Losses	[2]	(21)		(13)
Fair Value, Total		3,197		2,881
Net assets related to pending purchases of securities		10
Fair value of securities in an unrealized loss position		142		185
Common/collective trust funds | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[3]	170		115
Fixed income securities Total Unrealized Gains	[3]	5		4
Fixed income securities Fair Value	[3]	175		119
Common/collective trust funds | Fixed Income | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[3]	58		51
Fixed income securities Fair Value	[3]	58		51
Equity securities: | U.S.
Investment Holdings [Line Items]
Equity securities Amortized Cost	[4]	1,756		1,807
Equity securities Total Unrealized Gains	[4]	2,948		2,451
Equity securities Total Unrealized Losses	[4]	(24)		(20)
Equity securities Fair Value	[4]	4,680		4,238
Equity securities: | U.S. | Virginia Electric and Power Company
Investment Holdings [Line Items]
Equity securities Amortized Cost	[5]	929		894
Equity securities Total Unrealized Gains	[5]	1,371		1,144
Equity securities Total Unrealized Losses	[5]	(21)		(11)
Equity securities Fair Value	[5]	2,279		2,027
Corporate debt instruments | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[3]	572		434
Fixed income securities Total Unrealized Gains	[3]	58		29
Fixed income securities Total Unrealized Losses	[3]	(1)		0
Fixed income securities Fair Value	[3]	629		463
Corporate debt instruments | Fixed Income | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[3]	315		241
Fixed income securities Total Unrealized Gains	[3]	33		15
Fixed income securities Fair Value	[3]	348		256
Government Securities | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[3]	1,119		1,108
Fixed income securities Total Unrealized Gains	[3]	66		39
Fixed income securities Total Unrealized Losses	[3]	(1)		(2)
Fixed income securities Fair Value	[3]	1,184		1,145
Government Securities | Fixed Income | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[3]	484		534
Fixed income securities Total Unrealized Gains	[3]	25		14
Fixed income securities Total Unrealized Losses	[3]	0		(2)
Fixed income securities Fair Value	[3]	509		546
Insurance Contracts | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total		237		214
Fixed income securities Fair Value		237		214
Cash equivalents and other
Investment Holdings [Line Items]
Cash equivalents and other, Amortized Cost		(8)	[6]	13
Cash equivalents and other, Total Unrealized Gains		4	[6]	0
Cash equivalents and other, Total Unrealized Losses		(1)	[6]	0
Cash equivalents and other, Fair Value		(5)	[6]	13
Cash equivalents and other | Virginia Electric and Power Company
Investment Holdings [Line Items]
Cash equivalents and other, Amortized Cost		3		1
Cash equivalents and other, Fair Value		$ 3		$ 1

[1] The fair value of securities in an unrealized loss position was $293 million and $298 million at December 31, 2020 and 2019, respectively.

[2] The fair value of securities in an unrealized loss position was $142 million and $185 million at December 31, 2020 and 2019, respectively.

[3] Unrealized gains and losses on fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2. Effective January 2020, changes in allowance for credit losses are included in other income.

[4] Unrealized gains and losses on equity securities are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.

[5] Unrealized gains and losses on equity securities are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2

[6] Includes pending purchases of securities of $49 million and $1 million at December 31, 2020 and 2019, respectively.