Goodwill - Narrative (Details) - GBP (£) £ in Millions	Mar. 31, 2026	Mar. 31, 2025
US Regulated
Disclosure of reconciliation of changes in goodwill [line items]
Growth rate used to extrapolate cash flow projections	2.40%	2.30%
UK Electricity Distribution
Disclosure of reconciliation of changes in goodwill [line items]
Growth rate used to extrapolate cash flow projections	1.60%	1.80%
UK Electricity Distribution Group
Disclosure of reconciliation of changes in goodwill [line items]
Discount rate applied to cash flow projections	5.20%	5.40%
UK Electricity Distribution DNO
Disclosure of reconciliation of changes in goodwill [line items]
Discount rate applied to cash flow projections	5.20%	5.30%
New York
Disclosure of reconciliation of changes in goodwill [line items]
Discount rate applied to cash flow projections	5.10%	6.30%
New England
Disclosure of reconciliation of changes in goodwill [line items]
Discount rate applied to cash flow projections	5.00%	6.20%
National Grid Ventures – US
Disclosure of reconciliation of changes in goodwill [line items]
Discount rate applied to cash flow projections	5.30%	6.70%
Accumulated impairment \| Goodwill
Disclosure of reconciliation of changes in goodwill [line items]
Intangible assets and goodwill	£ 0	£ 0

The discount rate applied to cash flow projections for a cash-generating unit (group of units). [Refer: Total for all cash-generating units [member]]

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The amount of intangible assets and goodwill held by the entity. [Refer: Goodwill; Intangible assets other than goodwill]

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