Financial risk management - Schedule of impacts of reasonably possible changes in significant level 3 assumptions (Details) - GBP (£) £ in Millions	Mar. 31, 2024	Mar. 31, 2023
Financing derivatives \| + 20 basis points change in Limited Price Inflation (LPI) market curve
Disclosure of detailed information about financial instruments [line items]
+20 basis points change in Limited Price Inflation (LPI) market curve	£ (41)	£ (53)
Financing derivatives \| - 20 basis points change in LPI market curve
Disclosure of detailed information about financial instruments [line items]
–20 basis points change in LPI market curve	41	51
Financing derivatives \| +20 basis points increase between RPI and Consumer Price Index (CPI)
Disclosure of detailed information about financial instruments [line items]
+20 basis points increase between RPI and Consumer Price Index (CPI)	37	43
Financing derivatives \| -20 basis points decrease between RPI and CPI
Disclosure of detailed information about financial instruments [line items]
-20 basis points decrease between RPI and CPI	(34)	(38)
Commodity contract derivatives \| 10% increase in commodity prices
Disclosure of detailed information about financial instruments [line items]
10% increase in commodity prices	4	5
Commodity contract derivatives \| 10% decrease in commodity prices
Disclosure of detailed information about financial instruments [line items]
10% decrease in commodity prices	(4)	(6)
Other \| +100 basis points change in discount rate
Disclosure of detailed information about financial instruments [line items]
+100 basis points change in discount rate	(7)	(9)
Other \| -100 basis points change in discount rate
Disclosure of detailed information about financial instruments [line items]
-100 basis points change in discount rate	9	10
Other \| +10% change in venture capital price
Disclosure of detailed information about financial instruments [line items]
+10% change in venture capital price	28	28
Other \| -10% change in venture capital price
Disclosure of detailed information about financial instruments [line items]
-10% change in venture capital price	£ (28)	£ (28)

- Definition

Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

+ References

No definition available.

+ Details

Name:	ifrs-full_DisclosureOfFinancialInstrumentsLineItems
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Sensitivity Analysis, 100 Basis Point Change Decrease In Discount Rate

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis100BasisPointChangeDecreaseInDiscountRate
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sensitivity Analysis, 100 Basis Point Change Increase In Discount Rate

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis100BasisPointChangeIncreaseInDiscountRate
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sensitivity Analysis, 10 Percent Decrease In Commodity Prices

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis10PercentDecreaseInCommodityPrices
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sensitivity Analysis, 10 Percent Decrease In Venture Capital Price

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis10PercentDecreaseInVentureCapitalPrice
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sensitivity Analysis, 10 Percent Increase In Commodity Prices

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis10PercentIncreaseInCommodityPrices
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sensitivity Analysis, 10 Percent Increase In Venture Capital Price

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis10PercentIncreaseInVentureCapitalPrice
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sensitivity Analysis, 20 Basis Point Change Decrease Between Retail Price Index And Consumer Price Index

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis20BasisPointChangeDecreaseBetweenRetailPriceIndexAndConsumerPriceIndex
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sensitivity Analysis, 20 Basis Point Change Decrease In Market Curve

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis20BasisPointChangeDecreaseInMarketCurve
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Sensitivity Analysis, 20 Basis Point Change Increase Between Retail Price Index And Consumer Price Index

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis20BasisPointChangeIncreaseBetweenRetailPriceIndexAndConsumerPriceIndex
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Sensitivity Analysis, 20 Basis Point Change Increase Market Curve

+ References

No definition available.

+ Details

Name:	nggtf_SensitivityAnalysis20BasisPointChangeIncreaseMarketCurve
Namespace Prefix:	nggtf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_FinancingDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_MarketRiskLimitedPriceInflationMarketCurveTwentyBasisPointsIncreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_MarketRiskLimitedPriceInflationMarketCurveTwentyBasisPointsDecreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_MarketRiskRetailPriceIndexConsumerPriceIndexTwentyBasisPointsIncreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_MarketRiskRetailPriceIndexConsumerPriceIndexTwentyBasisPointsDecreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_CommodityDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_CommodityPriceRiskTenPercentIncreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_CommodityPriceRiskTenPercentDecreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfFinancialInstrumentsAxis=nggtf_OtherFinancialInstrumentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_DiscountRateRiskFiftyBasisPointsIncreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_DiscountRateRiskFiftyBasisPointsDecreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_VentureCapitalPriceTenPercentIncreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_TypesOfRisksAxis=nggtf_VentureCapitalPriceTenPercentDecreaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: