Significant Accounting Policies (Reconciliation of Total Cash, Restricted Cash and Equivalents (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017		Sep. 30, 2017	Dec. 31, 2016
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents		$ 310	$ 120	[1]	$ 227	$ 261
Restricted cash and equivalents	[2]	77	65		74	61
Cash, restricted cash and equivalents shown in the Consolidated Statements of Cash Flows		387	185		301	322
Virginia Electric and Power Company
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents		22	14	[3]	16	11
Restricted cash and equivalents	[2]	10	10
Cash, restricted cash and equivalents shown in the Consolidated Statements of Cash Flows		32	24		16	11
Dominion Energy Gas Holdings, LLC
Cash Cash Equivalents And Restricted Cash [Line Items]
Cash and cash equivalents		4	4	[4]	13	23
Restricted cash and equivalents	[2]	22	26		29	20
Cash, restricted cash and equivalents shown in the Consolidated Statements of Cash Flows		$ 26	$ 30		$ 42	$ 43

[1] Dominion Energy’s Consolidated Balance Sheet at December 31, 2017 has been derived from the audited Consolidated Balance Sheet at that date.

[2] Restricted cash and equivalent balances are presented within other current assets in the Companies’ Consolidated Balance Sheets.

[3] Virginia Power’s Consolidated Balance Sheet at December 31, 2017 has been derived from the audited Consolidated Balance Sheet at that date.

[4] Dominion Energy Gas’ Consolidated Balance Sheet at December 31, 2017 has been derived from the audited Consolidated Balance Sheet at that date.