1001 - Document - Cover link:presentationLink link:definitionLink link:calculationLink 1002 - Statement - Statements of Net Assets Available For Benefits link:presentationLink link:definitionLink link:calculationLink 1003 - Statement - Statement of Changes in Net Assets Available for Benefits link:presentationLink link:definitionLink link:calculationLink 1004 - Disclosure - Description of Plan link:presentationLink link:definitionLink link:calculationLink 1005 - Disclosure - Summary of Significant Accounting Policies link:presentationLink link:definitionLink link:calculationLink 1006 - Disclosure - Fair Value of Investments link:presentationLink link:definitionLink link:calculationLink 1007 - Disclosure - Risks and Uncertainties link:presentationLink link:definitionLink link:calculationLink 1008 - Disclosure - Plan Termination link:presentationLink link:definitionLink link:calculationLink 1009 - Disclosure - Plan Tax Status link:presentationLink link:definitionLink link:calculationLink 1010 - Disclosure - Party-In-Interest And Related Party Transactions link:presentationLink link:definitionLink link:calculationLink 1011 - Disclosure - Reconciliation of Financial Statements to Form 5500 link:presentationLink link:definitionLink link:calculationLink 1012 - Disclosure - SCHEDULE H, LINE 4i—SCHEDULE OF ASSETS (HELD AT END OF YEAR) link:presentationLink link:definitionLink link:calculationLink 1013 - Disclosure - Summary of Significant Accounting Policies (Policies) link:presentationLink link:definitionLink link:calculationLink 1014 - Disclosure - Description of Plan (Table) link:presentationLink link:definitionLink link:calculationLink 1015 - Disclosure - Fair Value of Investments (Tables) link:presentationLink link:definitionLink link:calculationLink 1016 - Disclosure - Reconciliation of Financial Statements to Form 5500 (Tables) link:presentationLink link:definitionLink link:calculationLink 1017 - Disclosure - Description of Plan - Summary of contribution (Detail) link:presentationLink link:definitionLink link:calculationLink 1018 - Disclosure - Description of Plan - Summary of based on years of service (Detail) link:presentationLink link:definitionLink link:calculationLink 1019 - Disclosure - Description of Plan - Summary of contribution points (Detail) link:presentationLink link:definitionLink link:calculationLink 1020 - Disclosure - Description of Plan - Summary of contribution (Parenthetical) (Detail) link:presentationLink link:definitionLink link:calculationLink 1021 - Disclosure - Description of Plan - Additional Information (Detail) link:presentationLink link:definitionLink link:calculationLink 1022 - Disclosure - Summary of Significant Accounting Policies - Additional Information (Detail) link:presentationLink link:definitionLink link:calculationLink 1023 - Disclosure - Fair Value of Investments - Summary of the Plan's Assets at Fair Value (Detail) link:presentationLink link:definitionLink link:calculationLink 1024 - Disclosure - Plan Termination - Additional Information (Detail) link:presentationLink link:definitionLink link:calculationLink 1025 - Disclosure - Party-In-Interest And Related Party Transactions - Additional Information (Detail) link:presentationLink link:definitionLink link:calculationLink 1026 - Disclosure - Reconciliation of Financial Statements to Form 5500 - Summary of Reconciliation of Net Assets Available for Benefits Per the Financial Statements to From 5500 (Detail) link:presentationLink link:definitionLink link:calculationLink 1027 - Disclosure - Reconciliation of Financial Statements to Form 5500 - Summary of Reconciliation of the Change in Net Asset Per the Financial Statements to the Form 5500 (Detail) link:presentationLink link:definitionLink link:calculationLink 1028 - Disclosure - Schedule H, Line 4i- Schedule of Assets (Held at End of Year) (Details) link:presentationLink link:definitionLink link:calculationLink 1029 - Disclosure - Schedule H, Line 4i- Schedule of Assets (Held at End of Year) (Details) Alternate link:presentationLink link:definitionLink link:calculationLink Cover [Abstract] Document Information [Table] Legal Entity [Axis] Entity [Domain] EBP 007 [Member] Document Information [Line Items] Document Type Amendment Flag Document Period End Date Document Fiscal Year Focus Document Fiscal Period Focus Entity Registrant Name Entity Central Index Key Current Fiscal Year End Date EBP, Statement of Net Asset Available for Benefit [Abstract] EBP, Statement of Net Asset Available for Benefit [Table] EBP, Statement of Net Asset Available for Benefit [Line Items] EBP, Asset [Abstract] ASSETS EBP, Investment, Excluding Plan Interest in Master Trust, Fair Value INVESTMENTS (at Fair Value) EBP, Investment, Fair Value EBP, Receivable [Abstract] RECEIVABLES EBP, Note Receivable from Participant Notes Receivable from Participants EBP, Employer Contribution, Receivable Company Contributions EBP, Investment Income, Receivable Dividends EBP, Receivable Total Receivables EBP, Asset Total Assets EBP, Net Asset Available for Benefit NET ASSETS AVAILABLE FOR BENEFITS Beginning of Year End of Year Net Assets Available for Benefits per Financial Statements EBP, Statement of Change in Net Asset Available for Benefit [Abstract] EBP, Statement of Change in Net Asset Available for Benefit [Table] EBP, Statement of Change in Net Asset Available for Benefit [Line Items] EBP, Change in Net Asset Available for Benefit, Increase [Abstract] ADDITIONS: EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Income (Loss) on Investment [Abstract] INVESTMENT INCOME EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Gain (Loss) on Investment Net Appreciation in Fair Value of Investments EBP, Change in Net Asset Available for Benefit, Increase from Interest and Dividend Income on Investment Interest and Dividends EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Other Investment Income (Loss) Other Additions EBP, Change in Net Asset Available for Benefit, Increase (Decrease) from Investment Income (Loss), before Investment Expense Total Investment Income EBP, Change in Net Asset Available for Benefit, Increase from Contribution [Abstract] CONTRIBUTIONS EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Excluding Rollover Employee Contributions EBP, Change in Net Asset Available for Benefit, Increase from Employer Contribution, Cash and Noncash Company Contributions EBP, Change in Net Asset Available for Benefit, Increase from Participant Contribution, Rollover Employee Rollover EBP, Change in Net Asset Available for Benefit, Increase from Contribution Total Contributions EBP, Change in Net Asset Available for Benefit, Increase from Interest Income on Note Receivable from Participant INTEREST INCOME ON NOTES RECEIVABLE FROM PARTICIPANTS EBP, Change in Net Asset Available for Benefit, Increase Total Additions EBP, Change in Net Asset Available for Benefit, Decrease [Abstract] DEDUCTIONS: EBP, Change in Net Asset Available for Benefit, Decrease from Payment to Participant BENEFITS PAID TO PARTICIPANTS EBP, Change in Net Asset Available for Benefit, Decrease from Administrative Expense ADMINISTRATIVE EXPENSES EBP, Change in Net Asset Available for Benefit, Decrease Total Deductions EBP, Change in Net Asset Available for Benefit, Increase (Decrease) NET INCREASE Change in Net Assets Available for Benefits per Financial Statements EBP, Description of Plan and Accounting Change [Abstract] EBP, Description of Plan [Table] EBP, Description of Plan [Line Items] EBP, Description of Plan [Text Block] DESCRIPTION OF PLAN EBP, Accounting Policy, Accounting Standard Update, and Change in Accounting Principle [Abstract] EBP, Accounting Policy [Table] EBP, Accounting Policy [Line Items] EBP, Summary of Accounting Policy [Text Block] Summary of Significant Accounting Policies EBP, Investment, Fair Value and NAV [Abstract] EBP, Investment, Fair Value and NAV [Table] EBP, Investment, Fair Value and NAV [Line Items] EBP, Investment, Fair Value and NAV [Text Block] Fair Value of Investments EBP, Risk and Uncertainty [Abstract] EBP, Risk and Uncertainty [Table] EBP, Risk and Uncertainty [Line Items] EBP, Risk and Uncertainty [Text Block] Risks and Uncertainties Employee Benefit Plan Plan Termination [Abstract] Employee Benefit Plan Plan Termination [Table] Employee Benefit Plan Plan Termination [Line Items] EBP, Plan Termination [Text Block] Plan Termination EBP, Tax Status [Abstract] EBP, Tax Status [Table] EBP, Tax Status [Line Items] EBP, Tax Status [Text Block] Plan Tax Status EBP, Related Party and Party-in-Interest Transactions [Abstract] EBP, Related Party and Party-in-Interest Transactions [Table] EBP, Related Party and Party-in-Interest Transactions [Line Items] EBP, Related Party and Party-in-Interest Transactions [Text Block] Party-In-Interest And Related Party Transactions EBP, Reconciliation of Financial Statement to Form 5500 [Abstract] EBP, Reconciliation of Financial Statement to Form 5500 [Table] EBP, Reconciliation of Financial Statement to Form 5500 [Line Items] EBP, Reconciliation of Financial Statement to Form 5500 [Text Block] Reconciliation of Financial Statements to Form 5500 EBP, Schedule of Asset Held for Investment [Abstract] EBP, Schedule of Asset Held for Investment [Table] EBP, Schedule of Asset Held for Investment [Line Items] EBP, Schedule of Asset Held for Investment [Table Text Block] SCHEDULE H, LINE 4i—SCHEDULE OF ASSETS (HELD AT END OF YEAR) EBP, Basis of Accounting [Policy Text Block] Basis of Accounting EBP, Use of Estimate [Policy Text Block] Use of Estimates EBP, Investment [Policy Text Block] Investment Valuation and Income Recognition EBP, Note Receivable from Participant [Policy Text Block] Notes Receivable from Participants EBP, Payment to Participant [Policy Text Block] Benefit Payments EBP, Expense [Policy Text Block] Administrative Expenses EBP, Subsequent Event [Policy Text Block] Subsequent Events EBP, Contribution [Abstract] EBP, Contribution [Table] EBP, Contribution [Line Items] EBP, Contribution [Table Text Block] Summary of contribution Employee Benefit Plan Contribution based On Years Of Service [Table Text Block] Summary of based on years of service Employee benefit plan contribution based on years of service. Employer Contribution Points [Table Text Block] Summary of contribution points Employer contribution points. EBP, Investment, Fair Value and NAV [Table Text Block] Summary of the Plan's Assets at Fair Value EBP, Reconciliation of Financial Statement to Form 5500 [Table Text Block] Summary of Reconciliation of Net Assets Available for Benefits Per the Financial Statements to From 5500 Employee Benefit Plan Reconciliation Of The Changes In Net Assets As Per Form 5500 [Table Text Block] Summary of Reconciliation of the Change in Net Asset Per the Financial Statements to the Form 5500 Employee benefit plan reconciliation of the changes in net assets as per form 5500. Employer Core Contribution Percentage Employer core contribution percentage Employer core contribution percentage. Participant Category [Axis] Participant Category [Domain] Collectively Bargained Participants [Member] Contribution Points Range [Axis] Contribution Points Range [Domain] Points Zero And Thirty Four [Member] 0 Less Than 34 [Member] Points Thirty Five To Forty Four [Member] 35 - 44 [Member] Points Forty Five To Fifty Four [Member] 45 - 54 [Member] Points Fifty Five To Sixty Four [Member] 55 - 64 [Member] Points Sixty Five To Seventy Four [Member] 65 - 74 [Member] Points Seventy Five To Eighty Four [Member] 75 - 84 [Member] Points Eighty Five And More [Member] 85 +[Member] Employee Benefit Plan Contribution based On Years Of Service [Table] Years Of Service [Axis] Years Of Service [Domain] Zero To Four Years Of Service [Member] 0 - 4 [Member] Five To Nine Years Of Service [Member] 5 - 9 [Member] Ten To fourteen Years Of Service [Member] 10 - 14 [Member] Fifteen To Nineteen Years Of Service [Member] 15 - 19 [Member] Twenty And More Years Of Service [Member] 20 + [Member] Employer Contribution Points [Table] Employer Contribution Points [Line Items] Non Collectively Bargained Participants [Member] Points Less Than Forty Five [Member] Less than 45 [Member] Points Seventy Five And More [Member] 75 + [Member] Employer Contribution Percentage Of Decrease In Pay Credit Employer contribution percentage of decrease in pay credit. Thirty Years Of Service [Member] 30 years of service [Member] EBP, Participant Contribution, Automatic, Deferral Rate EBP, participant contribution, automatic, deferral rate Employee Benefit Plan Participant Contribution Year Age Employee benefit plan participant contribution year age Employee benefit plan participant contribution year age. Participant Contribution Auto Increase Minimum Percentage Participant contribution auto increase minimum percentage Participant contribution auto increase minimum percentage. Participant Contribution Auto Increase Maximum Percentage Participant contribution auto increase maximum percentage Participant contribution auto increase maximum percentage. EBP, Employer Contribution, Matching Percentage EBP, employer contribution, matching percentage EBP, Employer Contribution, Matching Percentage EBP, Employer Contribution, Participant Compensation Matched, Percentage EBP, employer contribution, participant compensation matched, percentage EBP, Employer Contribution, Participant Compensation Matched, Percentage Participant Contribution Rollover From Qualified Pension Plan Participant contribution rollover from qualified pension plan Participant contribution rollover from qualified pension plan. Employee Benefit Plan Employer Matching And Core Contributions Percentage Employee benefit plan employer matching and core contributions percentage Employee benefit plan employer matching and core contributions percentage. EBP, Note Receivable from Participant, Account, Minimum Borrowing, Amount Employee benefit plan, note receivable from participant, account, minimum borrowing, amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, Amount Employee benefit plan, note receivable from participant, account, maximum borrowing, amount EBP, Note Receivable from Participant, Account, Maximum Borrowing, to Total Account, Percentage Employee benefit plan, note receivable from participant, account, maximum borrowing, to total account, percentage Employee Benefit Plan Note Receivable From Participant Account Loan Period Employee benefit plan note receivable from participant account loan period Employee benefit plan note receivable from participant account loan period. Cumulative Deemed Distributions Cumulative deemed distributions Cumulative deemed distributions. EBP, Forfeited Nonvested Account EBP, forfeited nonvested account EBP, Forfeited Nonvested Account, Decrease from Employer Contribution EBP, forfeited nonvested account, decrease from employer contribution Employee Benefit Plan Distrubution Of Benefit Vesting Balance Employee benefit plan distrubution of benefit vesting balance Employee benefit plan distrubution of benefit vesting balance. Three Years Of Service [Member] Statistical Measurement [Axis] Statistical Measurement [Domain] Minimum [Member] Maximum [Member] Subsequent Event Type [Axis] Subsequent Event Type [Domain] Subsequent Event [Member] Investment Type [Axis] Investments [Domain] Common Collective Trust [Member] Collective Trust Funds Common Stock [Member] Common Stock EBP, Self-Directed Brokerage Account [Member] Self-Directed Brokerage Fund Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Domain] Fair Value, Inputs, Level 1 [Member] Level 1 Fair Value, Inputs, Level 2 [Member] Level 2 Fair Value, Inputs, Level 3 [Member] Level 3 Plan Termination Vesting Percentage Plan Termination Vesting Percentage Plan termination vesting percentage. EBP, Investment, Number of Shares EBP, Investment, Number of Shares Administrative Fee Expense Administrative Fees Expense Administrative fee expense. EBP, Related Party and Party-in-Interest [Axis] EBP, Related Party and Party-in-Interest [Domain] EBP, Employer, Common Stock [Member] EBP, Trustee [Member] EBP, Reconciliation to Form 5500, Net Asset Available for Benefit, Deemed Distribution Deemed Distributions EBP, Form 5500 Caption, Net Assets Net Assets Available for Benefits per Form 5500 EBP, Reconciliation to Form 5500, Payment to Participant, Increase (Decrease) from Deemed Distribution Change in Deemed Distributions EBP, Form 5500 Caption, Benefit Payment and Payments to Provide Benefits Change in Net Assets Available for Benefits per Form 5500 Investment, Identifier [Axis] Investment, Issuer Name [Axis] Investment, Issuer Name [Domain] National Grid American Depository Receipt (ADR) Fund [Member] EBP, Participant-Directed and Nonparticipant-Directed Status [Axis] EBP, Participant-Directed and Nonparticipant-Directed Status [Domain] EBP, Participant-Directed [Member]