Investments (Equity and Debt Securities and Cash Equivalents and Cost Method Investments in Decommissioning Trust Funds) (Detail) - USD ($) $ in Millions		3 Months Ended	9 Months Ended	12 Months Ended
		Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings [Line Items]
Equity securities Total Unrealized Gains		$ 243	$ 267
Fixed income securities Fair Value		1,589	1,589
Amortized Cost, Total		3,316	3,316	$ 3,200
Total Unrealized Gains		2,144	2,144	1,899
Total Unrealized Losses	[1]	(36)	(36)	(6)
Fair Value, Total		5,424	5,424	5,093
Virginia Electric and Power Company
Investment Holdings [Line Items]
Equity securities Total Unrealized Gains		106	118
Fixed income securities Fair Value		771	771
Amortized Cost, Total		1,620	1,620	1,549
Total Unrealized Gains		971	971	852
Total Unrealized Losses	[2]	(18)	(18)	(2)
Fair Value, Total		2,573	2,573	2,399
Fixed Income [Member]
Investment Holdings [Line Items]
Cash equivalents and other Amorized Cost	[3]	7	7	34
Cash equivalents and other Fair Value	[3]	7	7	34
Cash Equivalents and Other [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Cash equivalents and other Amorized Cost	[4]	1	1	22
Cash equivalents and other Fair Value	[4]	1	1	22
Cost Method Investments [Member]
Investment Holdings [Line Items]
Amortized Cost, Total				68
Fair Value, Total				68
Cost Method Investments [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Amortized Cost, Total				68
Fair Value, Total				68
Common/collective trust funds | Fixed Income [Member]
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[5]	82	82	60
Fixed income securities Fair Value	[5]	82	82	60
Common/collective trust funds | Fixed Income [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[6]	48	48	27
Fixed income securities Fair Value	[6]	48	48	27
Corporate debt instruments | Fixed Income [Member]
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[5]	438	438	430
Fixed income securities Total Unrealized Gains	[5]		6	15
Fixed income securities Total Unrealized Losses	[5]		(7)	(1)
Fixed income securities Fair Value	[5]	437	437	444
Corporate debt instruments | Fixed Income [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[6]	220	220	216
Fixed income securities Total Unrealized Gains	[6]		2	8
Fixed income securities Total Unrealized Losses	[6]		(4)
Fixed income securities Fair Value	[6]	218	218	224
Government Securities [Member] | Fixed Income [Member]
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[5]	1,079	1,079	1,039
Fixed income securities Total Unrealized Gains	[5]		10	27
Fixed income securities Total Unrealized Losses	[5]		(19)	(5)
Fixed income securities Fair Value	[5]	1,070	1,070	1,061
Government Securities [Member] | Fixed Income [Member] | Virginia Electric and Power Company
Investment Holdings [Line Items]
Fixed income securities Amortized Cost, Total	[6]	509	509	482
Fixed income securities Total Unrealized Gains	[6]		4	13
Fixed income securities Total Unrealized Losses	[6]		(8)	(2)
Fixed income securities Fair Value	[6]	505	505	493
Equity securities: | U.S.
Investment Holdings [Line Items]
Equity securities Amortized Cost,	[7]	1,710	1,710	1,569
Equity securities Total Unrealized Gains	[7]		2,128	1,857
Equity securities Total Unrealized Losses	[7]		(10)
Equity securities Fair Value	[7]	3,828	3,828	3,426
Equity securities: | U.S. | Virginia Electric and Power Company
Investment Holdings [Line Items]
Equity securities Amortized Cost,	[7]	842	842	734
Equity securities Total Unrealized Gains	[7]		965	831
Equity securities Total Unrealized Losses	[7]		(6)
Equity securities Fair Value	[7]	$ 1,801	$ 1,801	$ 1,565

[1] The fair value of securities in an unrealized loss position was $1.1 billion and $565 million at September 30, 2018 and December 31, 2017, respectively.

[2] The fair value of securities in an unrealized loss position was $539 million and $234 million at September 30, 2018 and December 31, 2017, respectively.

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

[4] Includes pending sales of securities of $1 million and $6 million at September 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.