Investments (Equity and Fixed Income Securities, Insurance Contracts and Cash Equivalents in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Mar. 31, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Fixed income securities Fair Value		$ 1,747
Amortized Cost, Total		3,715		$ 3,691
Total Unrealized Gains		1,784		2,523
Total Unrealized Losses	[1]	(165)		(22)
Allowance for Credit Losses, Total		(21)
Fair Value, Total		5,313		6,192
Fair value of securities in an unrealized loss position		415		298
Net assets related to pending sales and purchases of securities		12		1
Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Fair Value		852
Amortized Cost, Total		1,737		1,721
Total Unrealized Gains		823		1,173
Total Unrealized Losses	[2]	(80)		(13)
Allowance for Credit Losses, Total		(12)
Fair Value, Total		2,468		2,881
Fair value of securities in an unrealized loss position		241		185
Net assets related to pending sales and purchases of securities		2
Fixed Income
Investment Holdings [Line Items]
Cash equivalents and other Amortized Cost	[3]			13
Cash equivalents and other Fair Value	[3]	(4)		13
Common/collective trust funds | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[4]	118		115
Fixed income securities Total Unrealized Gains	[4]	2		4
Fixed income securities Allowance for Credit Losses	[4]	(2)
Fixed income securities Fair Value	[4]	118		119
Common/collective trust funds | Fixed Income | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[4]	49		51
Fixed income securities Fair Value	[4]	49		51
Equity securities: | U.S.
Investment Holdings [Line Items]
Equity securities Amortized Cost	[5]	1,813		1,807
Equity securities Total Unrealized Gains	[5]	1,709		2,451
Equity securities Total Unrealized Losses	[5]	(161)		(20)
Equity securities Fair Value	[5]	3,361		4,238
Equity securities: | U.S. | Virginia Electric and Power Company
Investment Holdings [Line Items]
Equity securities Amortized Cost	[5]	898		894
Equity securities Total Unrealized Gains	[5]	796		1,144
Equity securities Total Unrealized Losses	[5]	(80)		(11)
Equity securities Fair Value	[5]	1,614		2,027
Corporate debt instruments | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[4]	491		434
Fixed income securities Total Unrealized Gains	[4]	18		29
Fixed income securities Allowance for Credit Losses	[4]	(13)
Fixed income securities Fair Value	[4]	496		463
Corporate debt instruments | Fixed Income | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[4]	284		241
Fixed income securities Total Unrealized Gains	[4]	9		15
Fixed income securities Allowance for Credit Losses	[4]	(8)
Fixed income securities Fair Value	[4]	285		256
Government Securities | Fixed Income
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[4]	1,084		1,108
Fixed income securities Total Unrealized Gains	[4]	55		39
Fixed income securities Total Unrealized Losses	[4]			(2)
Fixed income securities Allowance for Credit Losses	[4]	(6)
Fixed income securities Fair Value	[4]	1,133		1,145
Government Securities | Fixed Income | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[4]	504		534
Fixed income securities Total Unrealized Gains	[4]	18		14
Fixed income securities Total Unrealized Losses	[4]			(2)
Fixed income securities Allowance for Credit Losses	[4]	(4)
Fixed income securities Fair Value	[4]	518		546
Insurance Contracts
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total		209		214
Fixed income securities Fair Value		209		214
Cash equivalents and other | Virginia Electric and Power Company
Investment Holdings [Line Items]
Cash equivalents and other Amortized Cost		2	[6]	1
Cash equivalents and other Fair Value		$ 2	[6]	$ 1

[1] The fair value of securities in an unrealized loss position was $415 million and $298 million at March 31, 2020 and December 31, 2019, respectively.

[2] The fair value of securities in an unrealized loss position was $241 million and $185 million at March 31, 2020 and December 31, 2019, respectively.

[3] Includes pending purchases of securities of $12 million and $1 million at March 31, 2020 and December 31, 2019, respectively

[4] Unrealized gains and losses on fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability. Effective January 2020, changes in allowance for credit losses are included in other income (expense).

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

[6] Includes pending sales of securities of $2 million at March 31, 2020.