Commitment and Contingencies (Schedule of Long Term Purchase Commitments) (Parenthetical) (Detail) - Virginia Electric and Power Company - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Long-term Purchase Commitment [Line Items]
Present value of total commitment for capacity payments	$ 142
Purchased electric capacity
Long-term Purchase Commitment [Line Items]
Payments	50	$ 114	$ 248
Energy payments
Long-term Purchase Commitment [Line Items]
Payments	$ 42	$ 72	$ 126

The amount purchased during the period under an unrecorded unconditional purchase obligation (for example, under the take-or-pay or throughput contract).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 440
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=77896938&loc=d3e25336-109308

Name:	us-gaap_UnrecordedUnconditionalPurchaseObligationPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration