3.20.1 html 1751 929 1 true 374 0 false 20 false false R1.htm 0001000 - Document - Document And Entity Information Sheet http://www.nationalgrid.com/role/DocumentAndEntityInformation Document And Entity Information Cover 1 false false R2.htm 1001000 - Statement - Consolidated income statement Sheet http://www.nationalgrid.com/role/ConsolidatedIncomeStatement Consolidated income statement Statements 2 false false R3.htm 1002000 - Statement - Consolidated statement of comprehensive income Sheet http://www.nationalgrid.com/role/ConsolidatedStatementOfComprehensiveIncome Consolidated statement of comprehensive income Statements 3 false false R4.htm 1003000 - Statement - Consolidated statement of changes in equity Sheet http://www.nationalgrid.com/role/ConsolidatedStatementOfChangesInEquity Consolidated statement of changes in equity Statements 4 false false R5.htm 1004000 - Statement - Consolidated statement of financial position Sheet http://www.nationalgrid.com/role/ConsolidatedStatementOfFinancialPosition Consolidated statement of financial position Statements 5 false false R6.htm 1005000 - Statement - Consolidated cash flow statement Sheet http://www.nationalgrid.com/role/ConsolidatedCashFlowStatement Consolidated cash flow statement Statements 6 false false R7.htm 1005001 - Statement - Consolidated cash flow statement (Parenthetical) Sheet http://www.nationalgrid.com/role/ConsolidatedCashFlowStatementParenthetical Consolidated cash flow statement (Parenthetical) Statements 7 false false R8.htm 2106100 - Disclosure - Basis of preparation and recent accounting developments Sheet http://www.nationalgrid.com/role/BasisOfPreparationAndRecentAccountingDevelopments Basis of preparation and recent accounting developments Notes 8 false false R9.htm 2107100 - Disclosure - Segmental analysis Sheet http://www.nationalgrid.com/role/SegmentalAnalysis Segmental analysis Notes 9 false false R10.htm 2108100 - Disclosure - Revenue Sheet http://www.nationalgrid.com/role/Revenue Revenue Notes 10 false false R11.htm 2109100 - Disclosure - Operating costs Sheet http://www.nationalgrid.com/role/OperatingCosts Operating costs Notes 11 false false R12.htm 2110100 - Disclosure - Exceptional items and remeasurements Sheet http://www.nationalgrid.com/role/ExceptionalItemsAndRemeasurements Exceptional items and remeasurements Notes 12 false false R13.htm 2111100 - Disclosure - Finance income and costs Sheet http://www.nationalgrid.com/role/FinanceIncomeAndCosts Finance income and costs Notes 13 false false R14.htm 2112100 - Disclosure - Tax Sheet http://www.nationalgrid.com/role/Tax Tax Notes 14 false false R15.htm 2113100 - Disclosure - Earnings per share (EPS) Sheet http://www.nationalgrid.com/role/EarningsPerShareEps Earnings per share (EPS) Notes 15 false false R16.htm 2114100 - Disclosure - Dividends Sheet http://www.nationalgrid.com/role/Dividends Dividends Notes 16 false false R17.htm 2115100 - Disclosure - Discontinued operations and assets held for sale Sheet http://www.nationalgrid.com/role/DiscontinuedOperationsAndAssetsHeldForSale Discontinued operations and assets held for sale Notes 17 false false R18.htm 2116100 - Disclosure - Goodwill Sheet http://www.nationalgrid.com/role/Goodwill Goodwill Notes 18 false false R19.htm 2117100 - Disclosure - Other intangible assets Sheet http://www.nationalgrid.com/role/OtherIntangibleAssets Other intangible assets Notes 19 false false R20.htm 2118100 - Disclosure - Property, plant and equipment Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipment Property, plant and equipment Notes 20 false false R21.htm 2119100 - Disclosure - Other non-current assets Sheet http://www.nationalgrid.com/role/OtherNonCurrentAssets Other non-current assets Notes 21 false false R22.htm 2120100 - Disclosure - Financial and other investments Sheet http://www.nationalgrid.com/role/FinancialAndOtherInvestments Financial and other investments Notes 22 false false R23.htm 2121100 - Disclosure - Investments in joint ventures and associates Sheet http://www.nationalgrid.com/role/InvestmentsInJointVenturesAndAssociates Investments in joint ventures and associates Notes 23 false false R24.htm 2122100 - Disclosure - Derivative financial instruments Sheet http://www.nationalgrid.com/role/DerivativeFinancialInstruments Derivative financial instruments Notes 24 false false R25.htm 2123100 - Disclosure - Inventories and other current intangibles Sheet http://www.nationalgrid.com/role/InventoriesAndOtherCurrentIntangibles Inventories and other current intangibles Notes 25 false false R26.htm 2124100 - Disclosure - Trade and other receivables Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivables Trade and other receivables Notes 26 false false R27.htm 2125100 - Disclosure - Cash and cash equivalents Sheet http://www.nationalgrid.com/role/CashAndCashEquivalents Cash and cash equivalents Notes 27 false false R28.htm 2126100 - Disclosure - Borrowings Sheet http://www.nationalgrid.com/role/Borrowings Borrowings Notes 28 false false R29.htm 2127100 - Disclosure - Trade and other payables Sheet http://www.nationalgrid.com/role/TradeAndOtherPayables Trade and other payables Notes 29 false false R30.htm 2128100 - Disclosure - Contract liabilities Sheet http://www.nationalgrid.com/role/ContractLiabilities Contract liabilities Notes 30 false false R31.htm 2129100 - Disclosure - Other non-current liabilities Sheet http://www.nationalgrid.com/role/OtherNonCurrentLiabilities Other non-current liabilities Notes 31 false false R32.htm 2130100 - Disclosure - Pensions and other post retirement benefits Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefits Pensions and other post retirement benefits Notes 32 false false R33.htm 2131100 - Disclosure - Provisions Sheet http://www.nationalgrid.com/role/Provisions Provisions Notes 33 false false R34.htm 2132100 - Disclosure - Share capital Sheet http://www.nationalgrid.com/role/ShareCapital Share capital Notes 34 false false R35.htm 2133100 - Disclosure - Other equity reserves Sheet http://www.nationalgrid.com/role/OtherEquityReserves Other equity reserves Notes 35 false false R36.htm 2134100 - Disclosure - Net debt Sheet http://www.nationalgrid.com/role/NetDebt Net debt Notes 36 false false R37.htm 2135100 - Disclosure - Commitments and contingencies Sheet http://www.nationalgrid.com/role/CommitmentsAndContingencies Commitments and contingencies Notes 37 false false R38.htm 2136100 - Disclosure - Related party transactions Sheet http://www.nationalgrid.com/role/RelatedPartyTransactions Related party transactions Notes 38 false false R39.htm 2137100 - Disclosure - Financial risk management Sheet http://www.nationalgrid.com/role/FinancialRiskManagement Financial risk management Notes 39 false false R40.htm 2138100 - Disclosure - Borrowing facilities Sheet http://www.nationalgrid.com/role/BorrowingFacilities Borrowing facilities Notes 40 false false R41.htm 2139100 - Disclosure - Subsidiary undertakings, joint ventures and associates Sheet http://www.nationalgrid.com/role/SubsidiaryUndertakingsJointVenturesAndAssociates Subsidiary undertakings, joint ventures and associates Notes 41 false false R42.htm 2140100 - Disclosure - Sensitivities Sheet http://www.nationalgrid.com/role/Sensitivities Sensitivities Notes 42 false false R43.htm 2141100 - Disclosure - Additional disclosures in respect of guaranteed securities Sheet http://www.nationalgrid.com/role/AdditionalDisclosuresInRespectOfGuaranteedSecurities Additional disclosures in respect of guaranteed securities Notes 43 false false R44.htm 2142100 - Disclosure - Transition to new accounting standards Sheet http://www.nationalgrid.com/role/TransitionToNewAccountingStandards Transition to new accounting standards Notes 44 false false R45.htm 2143100 - Disclosure - Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC (Notes) Notes http://www.nationalgrid.com/role/AcquisitionOfGeronimoEnergyLlcAndEmeraldEnergyVentureLlcNotes Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC (Notes) Notes 45 false false R46.htm 2144100 - Disclosure - Post balance sheet events Sheet http://www.nationalgrid.com/role/PostBalanceSheetEvents Post balance sheet events Notes 46 false false R47.htm 2206201 - Disclosure - Basis of preparation and recent accounting developments (Policies) Sheet http://www.nationalgrid.com/role/BasisOfPreparationAndRecentAccountingDevelopmentsPolicies Basis of preparation and recent accounting developments (Policies) Policies 47 false false R48.htm 2307301 - Disclosure - Segmental analysis (Tables) Sheet http://www.nationalgrid.com/role/SegmentalAnalysisTables Segmental analysis (Tables) Tables http://www.nationalgrid.com/role/SegmentalAnalysis 48 false false R49.htm 2308301 - Disclosure - Revenue (Tables) Sheet http://www.nationalgrid.com/role/RevenueTables Revenue (Tables) Tables http://www.nationalgrid.com/role/Revenue 49 false false R50.htm 2309301 - Disclosure - Operating costs (Tables) Sheet http://www.nationalgrid.com/role/OperatingCostsTables Operating costs (Tables) Tables http://www.nationalgrid.com/role/OperatingCosts 50 false false R51.htm 2310301 - Disclosure - Exceptional items and remeasurements (Tables) Sheet http://www.nationalgrid.com/role/ExceptionalItemsAndRemeasurementsTables Exceptional items and remeasurements (Tables) Tables http://www.nationalgrid.com/role/ExceptionalItemsAndRemeasurements 51 false false R52.htm 2311301 - Disclosure - Finance income and costs (Tables) Sheet http://www.nationalgrid.com/role/FinanceIncomeAndCostsTables Finance income and costs (Tables) Tables http://www.nationalgrid.com/role/FinanceIncomeAndCosts 52 false false R53.htm 2312301 - Disclosure - Tax (Tables) Sheet http://www.nationalgrid.com/role/TaxTables Tax (Tables) Tables http://www.nationalgrid.com/role/Tax 53 false false R54.htm 2313301 - Disclosure - Earnings per share (EPS) (Tables) Sheet http://www.nationalgrid.com/role/EarningsPerShareEpsTables Earnings per share (EPS) (Tables) Tables http://www.nationalgrid.com/role/EarningsPerShareEps 54 false false R55.htm 2314301 - Disclosure - Dividends (Tables) Sheet http://www.nationalgrid.com/role/DividendsTables Dividends (Tables) Tables http://www.nationalgrid.com/role/Dividends 55 false false R56.htm 2315301 - Disclosure - Discontinued operations and assets held for sale (Tables) Sheet http://www.nationalgrid.com/role/DiscontinuedOperationsAndAssetsHeldForSaleTables Discontinued operations and assets held for sale (Tables) Tables http://www.nationalgrid.com/role/DiscontinuedOperationsAndAssetsHeldForSale 56 false false R57.htm 2316301 - Disclosure - Goodwill (Tables) Sheet http://www.nationalgrid.com/role/GoodwillTables Goodwill (Tables) Tables http://www.nationalgrid.com/role/Goodwill 57 false false R58.htm 2317301 - Disclosure - Other intangible assets (Tables) Sheet http://www.nationalgrid.com/role/OtherIntangibleAssetsTables Other intangible assets (Tables) Tables http://www.nationalgrid.com/role/OtherIntangibleAssets 58 false false R59.htm 2318301 - Disclosure - Property, plant and equipment (Tables) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentTables Property, plant and equipment (Tables) Tables http://www.nationalgrid.com/role/PropertyPlantAndEquipment 59 false false R60.htm 2319301 - Disclosure - Other non-current assets (Tables) Sheet http://www.nationalgrid.com/role/OtherNonCurrentAssetsTables Other non-current assets (Tables) Tables http://www.nationalgrid.com/role/OtherNonCurrentAssets 60 false false R61.htm 2320301 - Disclosure - Financial and other investments (Tables) Sheet http://www.nationalgrid.com/role/FinancialAndOtherInvestmentsTables Financial and other investments (Tables) Tables http://www.nationalgrid.com/role/FinancialAndOtherInvestments 61 false false R62.htm 2321301 - Disclosure - Investments in joint ventures and associates (Tables) Sheet http://www.nationalgrid.com/role/InvestmentsInJointVenturesAndAssociatesTables Investments in joint ventures and associates (Tables) Tables http://www.nationalgrid.com/role/InvestmentsInJointVenturesAndAssociates 62 false false R63.htm 2322301 - Disclosure - Derivative financial instruments (Tables) Sheet http://www.nationalgrid.com/role/DerivativeFinancialInstrumentsTables Derivative financial instruments (Tables) Tables http://www.nationalgrid.com/role/DerivativeFinancialInstruments 63 false false R64.htm 2323301 - Disclosure - Inventories and other current intangibles (Tables) Sheet http://www.nationalgrid.com/role/InventoriesAndOtherCurrentIntangiblesTables Inventories and other current intangibles (Tables) Tables http://www.nationalgrid.com/role/InventoriesAndOtherCurrentIntangibles 64 false false R65.htm 2324301 - Disclosure - Trade and other receivables (Tables) Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivablesTables Trade and other receivables (Tables) Tables http://www.nationalgrid.com/role/TradeAndOtherReceivables 65 false false R66.htm 2325301 - Disclosure - Cash and cash equivalents (Tables) Sheet http://www.nationalgrid.com/role/CashAndCashEquivalentsTables Cash and cash equivalents (Tables) Tables http://www.nationalgrid.com/role/CashAndCashEquivalents 66 false false R67.htm 2326301 - Disclosure - Borrowings (Tables) Sheet http://www.nationalgrid.com/role/BorrowingsTables Borrowings (Tables) Tables http://www.nationalgrid.com/role/Borrowings 67 false false R68.htm 2327301 - Disclosure - Trade and other payables (Tables) Sheet http://www.nationalgrid.com/role/TradeAndOtherPayablesTables Trade and other payables (Tables) Tables http://www.nationalgrid.com/role/TradeAndOtherPayables 68 false false R69.htm 2328301 - Disclosure - Contract liabilities (Tables) Sheet http://www.nationalgrid.com/role/ContractLiabilitiesTables Contract liabilities (Tables) Tables http://www.nationalgrid.com/role/ContractLiabilities 69 false false R70.htm 2329301 - Disclosure - Other non-current liabilities (Tables) Sheet http://www.nationalgrid.com/role/OtherNonCurrentLiabilitiesTables Other non-current liabilities (Tables) Tables http://www.nationalgrid.com/role/OtherNonCurrentLiabilities 70 false false R71.htm 2330301 - Disclosure - Pensions and other post retirement benefits (Tables) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsTables Pensions and other post retirement benefits (Tables) Tables http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefits 71 false false R72.htm 2331301 - Disclosure - Provisions (Tables) Sheet http://www.nationalgrid.com/role/ProvisionsTables Provisions (Tables) Tables http://www.nationalgrid.com/role/Provisions 72 false false R73.htm 2332301 - Disclosure - Share capital (Tables) Sheet http://www.nationalgrid.com/role/ShareCapitalTables Share capital (Tables) Tables http://www.nationalgrid.com/role/ShareCapital 73 false false R74.htm 2333301 - Disclosure - Other equity reserves (Tables) Sheet http://www.nationalgrid.com/role/OtherEquityReservesTables Other equity reserves (Tables) Tables http://www.nationalgrid.com/role/OtherEquityReserves 74 false false R75.htm 2334301 - Disclosure - Net debt (Tables) Sheet http://www.nationalgrid.com/role/NetDebtTables Net debt (Tables) Tables http://www.nationalgrid.com/role/NetDebt 75 false false R76.htm 2335301 - Disclosure - Commitments and contingencies (Tables) Sheet http://www.nationalgrid.com/role/CommitmentsAndContingenciesTables Commitments and contingencies (Tables) Tables http://www.nationalgrid.com/role/CommitmentsAndContingencies 76 false false R77.htm 2336301 - Disclosure - Related party transactions (Tables) Sheet http://www.nationalgrid.com/role/RelatedPartyTransactionsTables Related party transactions (Tables) Tables http://www.nationalgrid.com/role/RelatedPartyTransactions 77 false false R78.htm 2337301 - Disclosure - Financial risk management (Tables) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementTables Financial risk management (Tables) Tables http://www.nationalgrid.com/role/FinancialRiskManagement 78 false false R79.htm 2338301 - Disclosure - Borrowing facilities (Tables) Sheet http://www.nationalgrid.com/role/BorrowingFacilitiesTables Borrowing facilities (Tables) Tables http://www.nationalgrid.com/role/BorrowingFacilities 79 false false R80.htm 2340301 - Disclosure - Sensitivities (Tables) Sheet http://www.nationalgrid.com/role/SensitivitiesTables Sensitivities (Tables) Tables http://www.nationalgrid.com/role/Sensitivities 80 false false R81.htm 2341301 - Disclosure - Additional disclosures in respect of guaranteed securities (Tables) Sheet http://www.nationalgrid.com/role/AdditionalDisclosuresInRespectOfGuaranteedSecuritiesTables Additional disclosures in respect of guaranteed securities (Tables) Tables http://www.nationalgrid.com/role/AdditionalDisclosuresInRespectOfGuaranteedSecurities 81 false false R82.htm 2342301 - Disclosure - Transition to new accounting standards (Tables) Sheet http://www.nationalgrid.com/role/TransitionToNewAccountingStandardsTables Transition to new accounting standards (Tables) Tables http://www.nationalgrid.com/role/TransitionToNewAccountingStandards 82 false false R83.htm 2343301 - Disclosure - Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC (Tables) Sheet http://www.nationalgrid.com/role/AcquisitionOfGeronimoEnergyLlcAndEmeraldEnergyVentureLlcTables Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC (Tables) Tables http://www.nationalgrid.com/role/AcquisitionOfGeronimoEnergyLlcAndEmeraldEnergyVentureLlcNotes 83 false false R84.htm 2406402 - Disclosure - Basis of preparation and recent accounting developments - Going concern (Details) Sheet http://www.nationalgrid.com/role/BasisOfPreparationAndRecentAccountingDevelopmentsGoingConcernDetails Basis of preparation and recent accounting developments - Going concern (Details) Details 84 false false R85.htm 2406403 - Disclosure - Basis of preparation and recent accounting developments - Treatment of interests in Quadgas HoldCo Limited (Quadgas) (Details) Sheet http://www.nationalgrid.com/role/BasisOfPreparationAndRecentAccountingDevelopmentsTreatmentOfInterestsInQuadgasHoldcoLimitedQuadgasDetails Basis of preparation and recent accounting developments - Treatment of interests in Quadgas HoldCo Limited (Quadgas) (Details) Details 85 false false R86.htm 2406404 - Disclosure - Basis of preparation and recent accounting developments - Accounting policy choices (Details) Sheet http://www.nationalgrid.com/role/BasisOfPreparationAndRecentAccountingDevelopmentsAccountingPolicyChoicesDetails Basis of preparation and recent accounting developments - Accounting policy choices (Details) Details 86 false false R87.htm 2407402 - Disclosure - Segmental analysis - Narrative (Details) Sheet http://www.nationalgrid.com/role/SegmentalAnalysisNarrativeDetails Segmental analysis - Narrative (Details) Details 87 false false R88.htm 2407403 - Disclosure - Segmental analysis - Revenue (Details) Sheet http://www.nationalgrid.com/role/SegmentalAnalysisRevenueDetails Segmental analysis - Revenue (Details) Details 88 false false R89.htm 2407404 - Disclosure - Segmental analysis - Operating profit from continuing operations (Details) Sheet http://www.nationalgrid.com/role/SegmentalAnalysisOperatingProfitFromContinuingOperationsDetails Segmental analysis - Operating profit from continuing operations (Details) Details 89 false false R90.htm 2407405 - Disclosure - Segmental analysis - Reconcile total operating profit to profit before tax from continuing operations (Details) Sheet http://www.nationalgrid.com/role/SegmentalAnalysisReconcileTotalOperatingProfitToProfitBeforeTaxFromContinuingOperationsDetails Segmental analysis - Reconcile total operating profit to profit before tax from continuing operations (Details) Details 90 false false R91.htm 2407406 - Disclosure - Segmental analysis - Capital expenditure (Details) Sheet http://www.nationalgrid.com/role/SegmentalAnalysisCapitalExpenditureDetails Segmental analysis - Capital expenditure (Details) Details 91 false false R92.htm 2408402 - Disclosure - Revenue - Disaggregation of revenue (Details) Sheet http://www.nationalgrid.com/role/RevenueDisaggregationOfRevenueDetails Revenue - Disaggregation of revenue (Details) Details 92 false false R93.htm 2408403 - Disclosure - Revenue - Narrative (Details) Sheet http://www.nationalgrid.com/role/RevenueNarrativeDetails Revenue - Narrative (Details) Details 93 false false R94.htm 2409402 - Disclosure - Operating costs - Analysis of expenses by nature (Details) Sheet http://www.nationalgrid.com/role/OperatingCostsAnalysisOfExpensesByNatureDetails Operating costs - Analysis of expenses by nature (Details) Details 94 false false R95.htm 2409403 - Disclosure - Operating costs - Payroll costs (Details) Sheet http://www.nationalgrid.com/role/OperatingCostsPayrollCostsDetails Operating costs - Payroll costs (Details) Details 95 false false R96.htm 2409404 - Disclosure - Operating costs - Number of employees (Details) Sheet http://www.nationalgrid.com/role/OperatingCostsNumberOfEmployeesDetails Operating costs - Number of employees (Details) Details 96 false false R97.htm 2409405 - Disclosure - Operating costs - Key management personnel (Details) Sheet http://www.nationalgrid.com/role/OperatingCostsKeyManagementPersonnelDetails Operating costs - Key management personnel (Details) Details 97 false false R98.htm 2409406 - Disclosure - Operating costs - Auditors remuneration (Details) Sheet http://www.nationalgrid.com/role/OperatingCostsAuditorsRemunerationDetails Operating costs - Auditors remuneration (Details) Details 98 false false R99.htm 2410402 - Disclosure - Exceptional items and remeasurements (Details) Sheet http://www.nationalgrid.com/role/ExceptionalItemsAndRemeasurementsDetails Exceptional items and remeasurements (Details) Details http://www.nationalgrid.com/role/ExceptionalItemsAndRemeasurementsTables 99 false false R100.htm 2410403 - Disclosure - Exceptional items and remeasurements - Narrative (Details) Sheet http://www.nationalgrid.com/role/ExceptionalItemsAndRemeasurementsNarrativeDetails Exceptional items and remeasurements - Narrative (Details) Details 100 false false R101.htm 2411402 - Disclosure - Finance income and costs (Details) Sheet http://www.nationalgrid.com/role/FinanceIncomeAndCostsDetails Finance income and costs (Details) Details http://www.nationalgrid.com/role/FinanceIncomeAndCostsTables 101 false false R102.htm 2412404 - Disclosure - Tax - Tax charges (Details) Sheet http://www.nationalgrid.com/role/TaxTaxChargesDetails Tax - Tax charges (Details) Details 102 false false R103.htm 2412405 - Disclosure - Tax - Tax charged/(credited) to the consolidated statement of comprehensive income and equity (Details) Sheet http://www.nationalgrid.com/role/TaxTaxChargedCreditedToConsolidatedStatementOfComprehensiveIncomeAndEquityDetails Tax - Tax charged/(credited) to the consolidated statement of comprehensive income and equity (Details) Details 103 false false R104.htm 2412406 - Disclosure - Tax - Effective tax rate reconciliation (Details) Sheet http://www.nationalgrid.com/role/TaxEffectiveTaxRateReconciliationDetails Tax - Effective tax rate reconciliation (Details) Details 104 false false R105.htm 2412407 - Disclosure - Tax - Tax included within the statement of financial position (Details) Sheet http://www.nationalgrid.com/role/TaxTaxIncludedWithinStatementOfFinancialPositionDetails Tax - Tax included within the statement of financial position (Details) Details 105 false false R106.htm 2412408 - Disclosure - Tax - Narrative (Details) Sheet http://www.nationalgrid.com/role/TaxNarrativeDetails Tax - Narrative (Details) Details 106 false false R107.htm 2412409 - Disclosure - Tax - Deferred tax assets not recognised (Details) Sheet http://www.nationalgrid.com/role/TaxDeferredTaxAssetsNotRecognisedDetails Tax - Deferred tax assets not recognised (Details) Details 107 false false R108.htm 2413402 - Disclosure - Earnings per share (EPS) - Narrative (Details) Sheet http://www.nationalgrid.com/role/EarningsPerShareEpsNarrativeDetails Earnings per share (EPS) - Narrative (Details) Details http://www.nationalgrid.com/role/EarningsPerShareEpsTables 108 false false R109.htm 2413403 - Disclosure - Earnings per share (EPS) - Schedule of earnings per share (Details) Sheet http://www.nationalgrid.com/role/EarningsPerShareEpsScheduleOfEarningsPerShareDetails Earnings per share (EPS) - Schedule of earnings per share (Details) Details http://www.nationalgrid.com/role/EarningsPerShareEpsTables 109 false false R110.htm 2414402 - Disclosure - Dividends (Details) Sheet http://www.nationalgrid.com/role/DividendsDetails Dividends (Details) Details http://www.nationalgrid.com/role/DividendsTables 110 false false R111.htm 2414403 - Disclosure - Dividends - Narrative (Details) Sheet http://www.nationalgrid.com/role/DividendsNarrativeDetails Dividends - Narrative (Details) Details 111 false false R112.htm 2415402 - Disclosure - Discontinued operations and assets held for sale - Narrative (Details) Sheet http://www.nationalgrid.com/role/DiscontinuedOperationsAndAssetsHeldForSaleNarrativeDetails Discontinued operations and assets held for sale - Narrative (Details) Details http://www.nationalgrid.com/role/DiscontinuedOperationsAndAssetsHeldForSaleTables 112 false false R113.htm 2415403 - Disclosure - Discontinued operations and assets held for sale - Summary income statement and statement of comprehensive income (Details) Sheet http://www.nationalgrid.com/role/DiscontinuedOperationsAndAssetsHeldForSaleSummaryIncomeStatementAndStatementOfComprehensiveIncomeDetails Discontinued operations and assets held for sale - Summary income statement and statement of comprehensive income (Details) Details 113 false false R114.htm 2416402 - Disclosure - Goodwill - Disclosure of goodwill (Details) Sheet http://www.nationalgrid.com/role/GoodwillDisclosureOfGoodwillDetails Goodwill - Disclosure of goodwill (Details) Details 114 false false R115.htm 2416403 - Disclosure - Goodwill - Narrative (Details) Sheet http://www.nationalgrid.com/role/GoodwillNarrativeDetails Goodwill - Narrative (Details) Details 115 false false R116.htm 2417402 - Disclosure - Other intangible assets (Details) Sheet http://www.nationalgrid.com/role/OtherIntangibleAssetsDetails Other intangible assets (Details) Details http://www.nationalgrid.com/role/OtherIntangibleAssetsTables 116 false false R117.htm 2418402 - Disclosure - Property, plant and equipment - Useful economic lives (Details) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentUsefulEconomicLivesDetails Property, plant and equipment - Useful economic lives (Details) Details 117 false false R118.htm 2418403 - Disclosure - Property, plant and equipment - Sensitivity of gas assets (Details) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentSensitivityOfGasAssetsDetails Property, plant and equipment - Sensitivity of gas assets (Details) Details 118 false false R119.htm 2418404 - Disclosure - Property, plant and equipment - Disclosure of property plant and equipment (Details) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentDisclosureOfPropertyPlantAndEquipmentDetails Property, plant and equipment - Disclosure of property plant and equipment (Details) Details 119 false false R120.htm 2418405 - Disclosure - Property, plant and equipment - Disclosure of right-of-use assets (Details) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentDisclosureOfRightOfUseAssetsDetails Property, plant and equipment - Disclosure of right-of-use assets (Details) Details 120 false false R121.htm 2418406 - Disclosure - Property, plant and equipment - Narrative (Details) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentNarrativeDetails Property, plant and equipment - Narrative (Details) Details 121 false false R122.htm 2418407 - Disclosure - Property, plant and equipment - Information in relation to property, plant and equipment (Details) Sheet http://www.nationalgrid.com/role/PropertyPlantAndEquipmentInformationInRelationToPropertyPlantAndEquipmentDetails Property, plant and equipment - Information in relation to property, plant and equipment (Details) Details 122 false false R123.htm 2419402 - Disclosure - Other non-current assets (Details) Sheet http://www.nationalgrid.com/role/OtherNonCurrentAssetsDetails Other non-current assets (Details) Details http://www.nationalgrid.com/role/OtherNonCurrentAssetsTables 123 false false R124.htm 2420403 - Disclosure - Financial and other investments - Schedule of financial assets (Details) Sheet http://www.nationalgrid.com/role/FinancialAndOtherInvestmentsScheduleOfFinancialAssetsDetails Financial and other investments - Schedule of financial assets (Details) Details 124 false false R125.htm 2421402 - Disclosure - Investments in joint ventures and associates - Share of net assets, joint ventures and associates (Details) Sheet http://www.nationalgrid.com/role/InvestmentsInJointVenturesAndAssociatesShareOfNetAssetsJointVenturesAndAssociatesDetails Investments in joint ventures and associates - Share of net assets, joint ventures and associates (Details) Details 125 false false R126.htm 2421403 - Disclosure - Investments in joint ventures and associates - Narrative (Details) Sheet http://www.nationalgrid.com/role/InvestmentsInJointVenturesAndAssociatesNarrativeDetails Investments in joint ventures and associates - Narrative (Details) Details 126 false false R127.htm 2421404 - Disclosure - Investments in joint ventures and associates - Summarised financial information, joint venture and associate (Details) Sheet http://www.nationalgrid.com/role/InvestmentsInJointVenturesAndAssociatesSummarisedFinancialInformationJointVentureAndAssociateDetails Investments in joint ventures and associates - Summarised financial information, joint venture and associate (Details) Details 127 false false R128.htm 2422402 - Disclosure - Derivative financial instruments - Derivative financial instruments (Details) Sheet http://www.nationalgrid.com/role/DerivativeFinancialInstrumentsDerivativeFinancialInstrumentsDetails Derivative financial instruments - Derivative financial instruments (Details) Details 128 false false R129.htm 2422403 - Disclosure - Derivative financial instruments - Financing derivatives (Details) Sheet http://www.nationalgrid.com/role/DerivativeFinancialInstrumentsFinancingDerivativesDetails Derivative financial instruments - Financing derivatives (Details) Details 129 false false R130.htm 2422404 - Disclosure - Derivative financial instruments - Commodity derivatives (Details) Sheet http://www.nationalgrid.com/role/DerivativeFinancialInstrumentsCommodityDerivativesDetails Derivative financial instruments - Commodity derivatives (Details) Details 130 false false R131.htm 2423402 - Disclosure - Inventories and other current intangibles (Details) Sheet http://www.nationalgrid.com/role/InventoriesAndOtherCurrentIntangiblesDetails Inventories and other current intangibles (Details) Details http://www.nationalgrid.com/role/InventoriesAndOtherCurrentIntangiblesTables 131 false false R132.htm 2424402 - Disclosure - Trade and other receivables - Summary of trade and other receivables (Details) Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivablesSummaryOfTradeAndOtherReceivablesDetails Trade and other receivables - Summary of trade and other receivables (Details) Details 132 false false R133.htm 2424403 - Disclosure - Trade and other receivables - Narrative (Details) Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivablesNarrativeDetails Trade and other receivables - Narrative (Details) Details 133 false false R134.htm 2424404 - Disclosure - Trade and other receivables - Provision for impairment of receivables (Details) Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivablesProvisionForImpairmentOfReceivablesDetails Trade and other receivables - Provision for impairment of receivables (Details) Details 134 false false R135.htm 2424405 - Disclosure - Trade and other receivables - Trade receivables, accrued income and provision split by geography (Details) Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivablesTradeReceivablesAccruedIncomeAndProvisionSplitByGeographyDetails Trade and other receivables - Trade receivables, accrued income and provision split by geography (Details) Details 135 false false R136.htm 2424406 - Disclosure - Trade and other receivables - Average expected loss rates and gross balances for the retail customer receivables in our US operations (Details) Sheet http://www.nationalgrid.com/role/TradeAndOtherReceivablesAverageExpectedLossRatesAndGrossBalancesForRetailCustomerReceivablesInOurUsOperationsDetails Trade and other receivables - Average expected loss rates and gross balances for the retail customer receivables in our US operations (Details) Details 136 false false R137.htm 2425402 - Disclosure - Cash and cash equivalents (Details) Sheet http://www.nationalgrid.com/role/CashAndCashEquivalentsDetails Cash and cash equivalents (Details) Details http://www.nationalgrid.com/role/CashAndCashEquivalentsTables 137 false false R138.htm 2426402 - Disclosure - Borrowings - Current and non-current borrowings (Details) Sheet http://www.nationalgrid.com/role/BorrowingsCurrentAndNonCurrentBorrowingsDetails Borrowings - Current and non-current borrowings (Details) Details 138 false false R139.htm 2426403 - Disclosure - Borrowings - Borrowings repayment schedule (Details) Sheet http://www.nationalgrid.com/role/BorrowingsBorrowingsRepaymentScheduleDetails Borrowings - Borrowings repayment schedule (Details) Details 139 false false R140.htm 2426404 - Disclosure - Borrowings - Narrative (Details) Sheet http://www.nationalgrid.com/role/BorrowingsNarrativeDetails Borrowings - Narrative (Details) Details 140 false false R141.htm 2426405 - Disclosure - Borrowings - Lease obligations (Details) Sheet http://www.nationalgrid.com/role/BorrowingsLeaseObligationsDetails Borrowings - Lease obligations (Details) Details 141 false false R142.htm 2427402 - Disclosure - Trade and other payables (Details) Sheet http://www.nationalgrid.com/role/TradeAndOtherPayablesDetails Trade and other payables (Details) Details http://www.nationalgrid.com/role/TradeAndOtherPayablesTables 142 false false R143.htm 2428402 - Disclosure - Contract liabilities - Summary of Contract Liabilities (Details) Sheet http://www.nationalgrid.com/role/ContractLiabilitiesSummaryOfContractLiabilitiesDetails Contract liabilities - Summary of Contract Liabilities (Details) Details 143 false false R144.htm 2428403 - Disclosure - Contract liabilities - Significant Changes in Contract Liabilities (Details) Sheet http://www.nationalgrid.com/role/ContractLiabilitiesSignificantChangesInContractLiabilitiesDetails Contract liabilities - Significant Changes in Contract Liabilities (Details) Details 144 false false R145.htm 2429402 - Disclosure - Other non-current liabilities (Details) Sheet http://www.nationalgrid.com/role/OtherNonCurrentLiabilitiesDetails Other non-current liabilities (Details) Details http://www.nationalgrid.com/role/OtherNonCurrentLiabilitiesTables 145 false false R146.htm 2430402 - Disclosure - Pensions and other post retirement benefits - Narrative (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsNarrativeDetails Pensions and other post retirement benefits - Narrative (Details) Details 146 false false R147.htm 2430403 - Disclosure - Pensions and other post retirement benefits - Results of the most recent actuarial valuations (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsResultsOfMostRecentActuarialValuationsDetails Pensions and other post retirement benefits - Results of the most recent actuarial valuations (Details) Details 147 false false R148.htm 2430404 - Disclosure - Pensions and other post retirement benefits - Security arrangements (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsSecurityArrangementsDetails Pensions and other post retirement benefits - Security arrangements (Details) Details 148 false false R149.htm 2430405 - Disclosure - Pensions and other post retirement benefits - Actuarial assumptions (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsActuarialAssumptionsDetails Pensions and other post retirement benefits - Actuarial assumptions (Details) Details 149 false false R150.htm 2430406 - Disclosure - Pensions and other post retirement benefits - Amounts recognized in consolidated statement of financial position (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsAmountsRecognizedInConsolidatedStatementOfFinancialPositionDetails Pensions and other post retirement benefits - Amounts recognized in consolidated statement of financial position (Details) Details 150 false false R151.htm 2430407 - Disclosure - Pensions and other post retirement benefits - Amounts recognised in the income statement and statement of other comprehensive income (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsAmountsRecognisedInIncomeStatementAndStatementOfOtherComprehensiveIncomeDetails Pensions and other post retirement benefits - Amounts recognised in the income statement and statement of other comprehensive income (Details) Details 151 false false R152.htm 2430408 - Disclosure - Pensions and other post retirement benefits - Reconciliation of the net defined benefit liability (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsReconciliationOfNetDefinedBenefitLiabilityDetails Pensions and other post retirement benefits - Reconciliation of the net defined benefit liability (Details) Details 152 false false R153.htm 2430409 - Disclosure - Pensions and other post retirement benefits - Change in the present value of defined benefit obligations (including unfunded obligations) (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsChangeInPresentValueOfDefinedBenefitObligationsIncludingUnfundedObligationsDetails Pensions and other post retirement benefits - Change in the present value of defined benefit obligations (including unfunded obligations) (Details) Details 153 false false R154.htm 2430410 - Disclosure - Pensions and other post retirement benefits - Change in fair value of plan assets (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsChangeInFairValueOfPlanAssetsDetails Pensions and other post retirement benefits - Change in fair value of plan assets (Details) Details 154 false false R155.htm 2430411 - Disclosure - Pensions and other post retirement benefits - Asset allocation strategy (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsAssetAllocationStrategyDetails Pensions and other post retirement benefits - Asset allocation strategy (Details) Details 155 false false R156.htm 2430412 - Disclosure - Pensions and other post retirement benefits - Asset allocations (Details) Sheet http://www.nationalgrid.com/role/PensionsAndOtherPostRetirementBenefitsAssetAllocationsDetails Pensions and other post retirement benefits - Asset allocations (Details) Details 156 false false R157.htm 2431402 - Disclosure - Provisions - Schedule of changes in provisions (Details) Sheet http://www.nationalgrid.com/role/ProvisionsScheduleOfChangesInProvisionsDetails Provisions - Schedule of changes in provisions (Details) Details 157 false false R158.htm 2431403 - Disclosure - Provisions - Current and non-current provisions (Details) Sheet http://www.nationalgrid.com/role/ProvisionsCurrentAndNonCurrentProvisionsDetails Provisions - Current and non-current provisions (Details) Details 158 false false R159.htm 2431404 - Disclosure - Provisions - Environmental provision (Details) Sheet http://www.nationalgrid.com/role/ProvisionsEnvironmentalProvisionDetails Provisions - Environmental provision (Details) Details 159 false false R160.htm 2431405 - Disclosure - Provisions - Narrative (Details) Sheet http://www.nationalgrid.com/role/ProvisionsNarrativeDetails Provisions - Narrative (Details) Details 160 false false R161.htm 2432402 - Disclosure - Share capital - Schedule of share capital (Details) Sheet http://www.nationalgrid.com/role/ShareCapitalScheduleOfShareCapitalDetails Share capital - Schedule of share capital (Details) Details 161 false false R162.htm 2432403 - Disclosure - Share capital - Narrative (Details) Sheet http://www.nationalgrid.com/role/ShareCapitalNarrativeDetails Share capital - Narrative (Details) Details 162 false false R163.htm 2433402 - Disclosure - Other equity reserves (Details) Sheet http://www.nationalgrid.com/role/OtherEquityReservesDetails Other equity reserves (Details) Details http://www.nationalgrid.com/role/OtherEquityReservesTables 163 false false R164.htm 2434402 - Disclosure - Net debt - Reconciliation of net cash flow to movement in net debt (Details) Sheet http://www.nationalgrid.com/role/NetDebtReconciliationOfNetCashFlowToMovementInNetDebtDetails Net debt - Reconciliation of net cash flow to movement in net debt (Details) Details 164 false false R165.htm 2434403 - Disclosure - Net debt - Composition of net debt (Details) Sheet http://www.nationalgrid.com/role/NetDebtCompositionOfNetDebtDetails Net debt - Composition of net debt (Details) Details 165 false false R166.htm 2434404 - Disclosure - Net debt - Analysis of changes in net debt (Details) Sheet http://www.nationalgrid.com/role/NetDebtAnalysisOfChangesInNetDebtDetails Net debt - Analysis of changes in net debt (Details) Details 166 false false R167.htm 2434405 - Disclosure - Net debt - Reconciliation of cash flow from financing liabilities to cash flow statement (Details) Sheet http://www.nationalgrid.com/role/NetDebtReconciliationOfCashFlowFromFinancingLiabilitiesToCashFlowStatementDetails Net debt - Reconciliation of cash flow from financing liabilities to cash flow statement (Details) Details 167 false false R168.htm 2434406 - Disclosure - Net debt - Reconciliation of changes in liabilities arising from financing activities (Details) Sheet http://www.nationalgrid.com/role/NetDebtReconciliationOfChangesInLiabilitiesArisingFromFinancingActivitiesDetails Net debt - Reconciliation of changes in liabilities arising from financing activities (Details) Details 168 false false R169.htm 2435402 - Disclosure - Commitments and contingencies (Details) Sheet http://www.nationalgrid.com/role/CommitmentsAndContingenciesDetails Commitments and contingencies (Details) Details http://www.nationalgrid.com/role/CommitmentsAndContingenciesTables 169 false false R170.htm 2436402 - Disclosure - Related party transactions - Related party transactions table (Details) Sheet http://www.nationalgrid.com/role/RelatedPartyTransactionsRelatedPartyTransactionsTableDetails Related party transactions - Related party transactions table (Details) Details 170 false false R171.htm 2437402 - Disclosure - Financial risk management - Narrative (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementNarrativeDetails Financial risk management - Narrative (Details) Details 171 false false R172.htm 2437403 - Disclosure - Financial risk management - Treasury credit risk (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementTreasuryCreditRiskDetails Financial risk management - Treasury credit risk (Details) Details 172 false false R173.htm 2437404 - Disclosure - Financial risk management - Offsetting financial assets and liabilities (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementOffsettingFinancialAssetsAndLiabilitiesDetails Financial risk management - Offsetting financial assets and liabilities (Details) Details 173 false false R174.htm 2437405 - Disclosure - Financial risk management - Liquidity risk (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementLiquidityRiskDetails Financial risk management - Liquidity risk (Details) Details 174 false false R175.htm 2437406 - Disclosure - Financial risk management - Currency risk (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementCurrencyRiskDetails Financial risk management - Currency risk (Details) Details 175 false false R176.htm 2437407 - Disclosure - Financial risk management - Interest rate risk (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementInterestRateRiskDetails Financial risk management - Interest rate risk (Details) Details 176 false false R177.htm 2437408 - Disclosure - Financial risk management - Impact of hedge accounting in the primary financial statements (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementImpactOfHedgeAccountingInPrimaryFinancialStatementsDetails Financial risk management - Impact of hedge accounting in the primary financial statements (Details) Details 177 false false R178.htm 2437409 - Disclosure - Financial risk management - Effects of hedge accounting on financial position and performance for each type of hedge (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementEffectsOfHedgeAccountingOnFinancialPositionAndPerformanceForEachTypeOfHedgeDetails Financial risk management - Effects of hedge accounting on financial position and performance for each type of hedge (Details) Details 178 false false R179.htm 2437410 - Disclosure - Financial risk management - Fair value analysis (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementFairValueAnalysisDetails Financial risk management - Fair value analysis (Details) Details 179 false false R180.htm 2437411 - Disclosure - Financial risk management - Changes in level 3 financial instruments (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementChangesInLevel3FinancialInstrumentsDetails Financial risk management - Changes in level 3 financial instruments (Details) Details 180 false false R181.htm 2437412 - Disclosure - Financial risk management - Impacts of reasonably possible changes in significant level 3 assumptions (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementImpactsOfReasonablyPossibleChangesInSignificantLevel3AssumptionsDetails Financial risk management - Impacts of reasonably possible changes in significant level 3 assumptions (Details) Details 181 false false R182.htm 2437413 - Disclosure - Financial risk management - Capital risk management (Details) Sheet http://www.nationalgrid.com/role/FinancialRiskManagementCapitalRiskManagementDetails Financial risk management - Capital risk management (Details) Details 182 false false R183.htm 2438402 - Disclosure - Borrowing facilities - Narrative (Details) Sheet http://www.nationalgrid.com/role/BorrowingFacilitiesNarrativeDetails Borrowing facilities - Narrative (Details) Details 183 false false R184.htm 2438403 - Disclosure - Borrowing facilities - Schedule of Undrawn Committed Borrowing Facilities (Details) Sheet http://www.nationalgrid.com/role/BorrowingFacilitiesScheduleOfUndrawnCommittedBorrowingFacilitiesDetails Borrowing facilities - Schedule of Undrawn Committed Borrowing Facilities (Details) Details 184 false false R185.htm 2439401 - Disclosure - Subsidiary undertakings, joint ventures and associates - Subsidiaries (Details) Sheet http://www.nationalgrid.com/role/SubsidiaryUndertakingsJointVenturesAndAssociatesSubsidiariesDetails Subsidiary undertakings, joint ventures and associates - Subsidiaries (Details) Details 185 false false R186.htm 2439402 - Disclosure - Subsidiary undertakings, joint ventures and associates - Joint ventures (Details) Sheet http://www.nationalgrid.com/role/SubsidiaryUndertakingsJointVenturesAndAssociatesJointVenturesDetails Subsidiary undertakings, joint ventures and associates - Joint ventures (Details) Details 186 false false R187.htm 2439403 - Disclosure - Subsidiary undertakings, joint ventures and associates - Associates (Details) Sheet http://www.nationalgrid.com/role/SubsidiaryUndertakingsJointVenturesAndAssociatesAssociatesDetails Subsidiary undertakings, joint ventures and associates - Associates (Details) Details http://www.nationalgrid.com/role/SubsidiaryUndertakingsJointVenturesAndAssociates 187 false false R188.htm 2440402 - Disclosure - Sensitivities - Sensitivities on areas of estimation uncertainty (Details) Sheet http://www.nationalgrid.com/role/SensitivitiesSensitivitiesOnAreasOfEstimationUncertaintyDetails Sensitivities - Sensitivities on areas of estimation uncertainty (Details) Details 188 false false R189.htm 2440403 - Disclosure - Sensitivities - Sensitivities on financial instruments (Details) Sheet http://www.nationalgrid.com/role/SensitivitiesSensitivitiesOnFinancialInstrumentsDetails Sensitivities - Sensitivities on financial instruments (Details) Details 189 false false R190.htm 2441402 - Disclosure - Additional disclosures in respect of guaranteed securities - Narrative (Details) Sheet http://www.nationalgrid.com/role/AdditionalDisclosuresInRespectOfGuaranteedSecuritiesNarrativeDetails Additional disclosures in respect of guaranteed securities - Narrative (Details) Details http://www.nationalgrid.com/role/AdditionalDisclosuresInRespectOfGuaranteedSecuritiesTables 190 false false R191.htm 2441403 - Disclosure - Additional disclosures in respect of guaranteed securities - Combined Statement of financial position and income statement (Details) Sheet http://www.nationalgrid.com/role/AdditionalDisclosuresInRespectOfGuaranteedSecuritiesCombinedStatementOfFinancialPositionAndIncomeStatementDetails Additional disclosures in respect of guaranteed securities - Combined Statement of financial position and income statement (Details) Details 191 false false R192.htm 2442402 - Disclosure - Transition to new accounting standards - Narrative (Details) Sheet http://www.nationalgrid.com/role/TransitionToNewAccountingStandardsNarrativeDetails Transition to new accounting standards - Narrative (Details) Details 192 false false R193.htm 2442403 - Disclosure - Transition to new accounting standards - IFRS 16: Impact of transition (Details) Sheet http://www.nationalgrid.com/role/TransitionToNewAccountingStandardsIfrs16ImpactOfTransitionDetails Transition to new accounting standards - IFRS 16: Impact of transition (Details) Details 193 false false R194.htm 2442404 - Disclosure - Transition to new accounting standards - IFRS 9 and IFRS 15: Impact of transition (Details) Sheet http://www.nationalgrid.com/role/TransitionToNewAccountingStandardsIfrs9AndIfrs15ImpactOfTransitionDetails Transition to new accounting standards - IFRS 9 and IFRS 15: Impact of transition (Details) Details 194 false false R195.htm 2442405 - Disclosure - Transition to new accounting standards - IFRS 9: Financial Instruments (Details) Sheet http://www.nationalgrid.com/role/TransitionToNewAccountingStandardsIfrs9FinancialInstrumentsDetails Transition to new accounting standards - IFRS 9: Financial Instruments (Details) Details 195 false false R196.htm 2443402 - Disclosure - Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC - Fair values of the assets and liabilities recognised (Details) Sheet http://www.nationalgrid.com/role/AcquisitionOfGeronimoEnergyLlcAndEmeraldEnergyVentureLlcFairValuesOfAssetsAndLiabilitiesRecognisedDetails Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC - Fair values of the assets and liabilities recognised (Details) Details 196 false false R197.htm 2443403 - Disclosure - Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC - Narrative (Details) Sheet http://www.nationalgrid.com/role/AcquisitionOfGeronimoEnergyLlcAndEmeraldEnergyVentureLlcNarrativeDetails Acquisition of Geronimo Energy LLC and Emerald Energy Venture LLC - Narrative (Details) Details http://www.nationalgrid.com/role/AcquisitionOfGeronimoEnergyLlcAndEmeraldEnergyVentureLlcTables 197 false false R9999.htm Uncategorized Items - nggtf-20200331.xml Sheet http://xbrl.sec.gov/role/uncategorizedFacts Uncategorized Items - nggtf-20200331.xml Cover 198 false false All Reports Book All Reports nggtf-20200331.xml nggtf-20200331.xsd nggtf-20200331_cal.xml nggtf-20200331_def.xml nggtf-20200331_lab.xml nggtf-20200331_pre.xml http://xbrl.ifrs.org/taxonomy/2019-03-27/ifrs-full http://xbrl.sec.gov/dei/2019-01-31 http://fasb.org/srt/2019-01-31 http://xbrl.sec.gov/stpr/2018-01-31 http://xbrl.sec.gov/country/2017-01-31 http://xbrl.sec.gov/currency/2019-01-31 true true