Other intangible assets - Schedule of analysis of other intangible assets (Details) - GBP (£) £ in Millions	12 Months Ended
Mar. 31, 2024	Mar. 31, 2023
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	£ 3,604
Intangible assets other than goodwill, ending balance	3,431	£ 3,604
Regulatory licenses
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	1,714
Intangible assets other than goodwill, ending balance	1,714	1,714
Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	1,339
Intangible assets other than goodwill, ending balance	1,335	1,339
Assets in the course of construction
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	551
Intangible assets other than goodwill, ending balance	382	551
Gas Business Enablement system
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	370
Intangible assets other than goodwill, ending balance	320	370
Intangible assets other than goodwill, in service amount	£ 320	369
Amortised period (in years)	10 years
UK General Ledger system
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	£ 87
Intangible assets other than goodwill, ending balance	£ 81	87
Amortised period (in years)	10 years
Gross carrying amount
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	£ 5,341	4,659
Exchange adjustments	(51)	111
Additions	481	578
Disposals	(23)	(17)
Reclassifications	162	10
Reclassification to held for sale (note 10)	(711)
Intangible assets other than goodwill, ending balance	5,199	5,341
Gross carrying amount \| Regulatory licenses
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	1,714	1,714
Exchange adjustments	0	0
Additions	0	0
Disposals	0	0
Reclassifications	0	0
Reclassification to held for sale (note 10)	0
Intangible assets other than goodwill, ending balance	1,714	1,714
Gross carrying amount \| Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	3,066	2,075
Exchange adjustments	(45)	79
Additions	17	34
Disposals	(23)	(17)
Reclassifications	598	895
Reclassification to held for sale (note 10)	(520)
Intangible assets other than goodwill, ending balance	3,093	3,066
Gross carrying amount \| Assets in the course of construction
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	561	870
Exchange adjustments	(6)	32
Additions	464	544
Disposals	0	0
Reclassifications	(436)	(885)
Reclassification to held for sale (note 10)	(191)
Intangible assets other than goodwill, ending balance	392	561
Accumulated amortisation
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	(1,737)	(1,387)
Exchange adjustments	23	(51)
Amortisation charge for the year	(301)	(291)
Disposals	23	15
Reclassifications	(161)	(23)
Reclassification to held for sale (note 10)	385
Intangible assets other than goodwill, ending balance	(1,768)	(1,737)
Accumulated amortisation \| Regulatory licenses
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	0	0
Exchange adjustments	0	0
Amortisation charge for the year	0	0
Disposals	0	0
Reclassifications	0	0
Reclassification to held for sale (note 10)	0
Intangible assets other than goodwill, ending balance	0	0
Accumulated amortisation \| Software
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	(1,727)	(1,377)
Exchange adjustments	23	(51)
Amortisation charge for the year	(301)	(291)
Disposals	23	15
Reclassifications	(161)	(23)
Reclassification to held for sale (note 10)	385
Intangible assets other than goodwill, ending balance	(1,758)	(1,727)
Accumulated amortisation \| Assets in the course of construction
Changes in intangible assets other than goodwill [abstract]
Intangible assets other than goodwill, beginning balance	(10)	(10)
Exchange adjustments	0	0
Amortisation charge for the year	0	0
Disposals	0	0
Reclassifications	0	0
Reclassification to held for sale (note 10)	0
Intangible assets other than goodwill, ending balance	£ (10)	£ (10)

The amount of additions to intangible assets other than goodwill, other than those acquired through business combinations. [Refer: Business combinations [member]; Intangible assets other than goodwill]

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph e
-Clause i
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_e_i&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_AdditionsOtherThanThroughBusinessCombinationsIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of amortisation of intangible assets other than goodwill. [Refer: Depreciation and amortisation expense; Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph e
-Clause vi
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_e_vi&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_AmortisationIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	duration

- References

No definition available.

+ Details

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

- Definition

The decrease in intangible assets other than goodwill resulting from classification as held for sale or inclusion in a disposal group classified as held for sale. [Refer: Disposal groups classified as held for sale [member]; Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph e
-Clause ii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_e_ii&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_DecreaseThroughClassifiedAsHeldForSaleIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The decrease in intangible assets other than goodwill resulting from disposals. [Refer: Intangible assets other than goodwill]

+ References

+ Details

Name:	ifrs-full_DisposalsIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The increase (decrease) in intangible assets other than goodwill resulting from net exchange differences arising on the translation of the financial statements from the functional currency into a different presentation currency, including the translation of a foreign operation into the presentation currency of the reporting entity. [Refer: Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph e
-Clause vii
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_e_vii&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The increase (decrease) in intangible assets other than goodwill resulting from transfers. [Refer: Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph e
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_e&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_IncreaseDecreaseThroughTransfersIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The amount of identifiable non-monetary assets without physical substance. This amount does not include goodwill. [Refer: Goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph e
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_e&doctype=Standard
-URIDate 2023-03-23

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 1
-IssueDate 2023-01-01
-Paragraph 54
-Subparagraph c
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=1&code=ifrs-tx-2023-en-r&anchor=para_54_c&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_IntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The useful life, measured as period of time, used for intangible assets other than goodwill. [Refer: Intangible assets other than goodwill]

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name IAS
-Number 38
-IssueDate 2023-01-01
-Paragraph 118
-Subparagraph a
-URI https://taxonomy.ifrs.org/xifrs-link?type=IAS&num=38&code=ifrs-tx-2023-en-r&anchor=para_118_a&doctype=Standard
-URIDate 2023-03-23

+ Details

Name:	ifrs-full_UsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	ifrs-full_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Intangible Assets Other Than Goodwill, In Service

+ References

No definition available.

+ Details

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

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis=ifrs-full_LicencesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis=ifrs-full_ComputerSoftwareMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis=ifrs-full_ConstructionInProgressMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis=nggtf_GasBusinessEnablementSystemMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis=nggtf_UKGeneralLedgerSystemMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis=ifrs-full_GrossCarryingAmountMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis=ifrs-full_AccumulatedDepreciationAndAmortisationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: