Investments (Equity and Debt Securities and Cash Equivalents and Cost Method Investments in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions		Jun. 30, 2018		Dec. 31, 2017
Investment Holdings [Line Items]
Amortized Cost, Total		$ 3,277		$ 3,200
Total Unrealized Gains		1,918		1,899
Total Unrealized Losses	[1]	(36)		(6)
Fair Value		5,159		5,093
Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Total		1,594		1,549
Total Unrealized Gains		871		852
Total Unrealized Losses	[2]	(16)		(2)
Fair Value		2,449		2,399
Cash Equivalents and Other [Member]
Investment Holdings [Line Items]
Amortized Cost, Total	[3]	5		34
Fair Value	[3]	5		34
Cash Equivalents and Other [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Other Investments	[4]	7		22
Fair Value	[4]	7		22
Cost Method Investments [Member]
Investment Holdings [Line Items]
Amortized Cost, Total				68
Fair Value				68
Cost Method Investments [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Other Investments				68
Fair Value				68
Common/collective trust funds | Fixed Income [Member]
Investment Holdings [Line Items]
Amortized Cost, Total	[5]	72		60
Fair Value	[5]	72		60
Common/collective trust funds | Fixed Income [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Total	[6]	32		27
Fair Value	[6]	32		27
Corporate debt instruments | Fixed Income [Member]
Investment Holdings [Line Items]
Amortized Cost, Total	[5]	447		430
Total Unrealized Gains	[5]	6		15
Total Unrealized Losses	[5]	(8)		(1)
Fair Value	[5]	445		444
Corporate debt instruments | Fixed Income [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Total	[6]	228		216
Total Unrealized Gains	[6],[7]	3		8
Total Unrealized Losses	[6],[7]	(4)
Fair Value	[6]	227		224
Government Securities [Member] | Fixed Income [Member]
Investment Holdings [Line Items]
Amortized Cost, Total	[5]	1,058		1,039
Total Unrealized Gains	[5]	16		27
Total Unrealized Losses	[5]	(14)		(5)
Fair Value	[5]	1,060		1,061
Government Securities [Member] | Fixed Income [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Total	[6]	490		482
Total Unrealized Gains	[6],[7]	7		13
Total Unrealized Losses	[6],[7]	(5)		(2)
Fair Value	[6]	492		493
Equity securities: | U.S.
Investment Holdings [Line Items]
Amortized Cost, Total		1,695	[8]	1,569	[5]
Total Unrealized Gains		1,896	[8]	1,857	[5]
Total Unrealized Losses	[8]	(14)
Fair Value		3,577	[8]	3,426	[5]
Equity securities: | U.S. | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Total	[8]	837		734
Total Unrealized Gains	[7],[8]	861		831
Total Unrealized Losses	[7],[8]	(7)
Fair Value	[8]	$ 1,691		$ 1,565

[1] The fair value of securities in an unrealized loss position was $900 million and $565 million at June 30, 2018 and December 31, 2017, respectively.

[2] The fair value of securities in an unrealized loss position was $414 million and $234 million at June 30, 2018 and December 31, 2017, respectively.

[3] Includes pending purchases of securities of $3 million at June 30, 2018 and pending sales of securities of $5 million at December 31, 2017.

[4] Includes pending sales of securities of $5 million and $6 million at June 30, 2018 and December 31, 2017, respectively.

[5] Unrealized gains and losses on equity securities (for 2017) and fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.

[6] Unrealized gains and losses on equity securities (for 2017) and fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.

[7] Effective January 2018, unrealized gains and losses on equity securities, including those previously classified as cost method investments, are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2. Unrealized gains and losses on equity securities (for 2017) and fixed income securities are included in AOCI and the nuclear decommissioning trust regulatory liability as discussed in Note 2.

[8] Effective January 2018, unrealized gains and losses on equity securities, including those previously classified as cost method investments, are included in other income and the nuclear decommissioning trust regulatory liability as discussed in Note 2.