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Employee Benefit Plans (Benefit Obligation in Excess of Plan Assets) (Detail) - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Pension Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefit obligation | $ 8,209 | $ 7,386
Fair value of plan assets | 6,103 | 5,340
Other Postretirement Benefits |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefit obligation | 191 | 470
Fair value of plan assets | 156 | 356
Other Postretirement Benefits | Dominion Energy Gas Holdings, LLC |  |
Defined Benefit Plan Disclosure [Line Items] |  |
Benefit obligation | 157 | 320
Fair value of plan assets | $ 143 | $ 299

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- Definition Amount of benefit obligation for defined benefit plan with benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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- Definition Fair value of plan assets for defined benefit plan with benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920 + Details
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