v3.26.1
Pensions and other post-retirement benefits - Narrative (Details)
£ in Millions
 1 Months Ended 12 Months Ended
Mar. 31, 2025
GBP (£)
plan
Jul. 31, 2025
GBP (£)
Mar. 31, 2026
GBP (£)
plan
arrangement
Mar. 31, 2025
GBP (£)
plan
Disclosure of defined benefit plans [line items]        
Independent actuarial funding valuation, period     3 years  
Number of plans showing funding shortfall | plan 1     1
Number of security arrangements | arrangement     2  
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     £ 120 £ 282
Restrictions on asset recognised £ 77   £ 244 77
Number of OPEB plans | plan     1  
Plan assets        
Disclosure of defined benefit plans [line items]        
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     £ 120 282
Pensions        
Disclosure of defined benefit plans [line items]        
Defined benefit plan, buy-in-transaction   £ 900    
Proportion of defined benefit plan, buy-in-transaction insured   0.75    
UK | Pensions        
Disclosure of defined benefit plans [line items]        
Employer contributions, recovery plan payments 18      
Employer contributions, excluding recovery plan payments     93 100
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     £ 93 112
Weighted average duration of defined benefit obligation     10 years  
Restrictions on asset recognised 0   £ 0 0
UK | Pensions | Maximum        
Disclosure of defined benefit plans [line items]        
Value of security arrangements     0  
UK | Pensions | Plan assets        
Disclosure of defined benefit plans [line items]        
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     £ 93 112
US | Pensions        
Disclosure of defined benefit plans [line items]        
Number of defined benefit plans | plan     4  
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     £ 19 27
Weighted average duration of defined benefit obligation     11 years  
Restrictions on asset recognised 0   £ 0 0
US | Pensions | Plan assets        
Disclosure of defined benefit plans [line items]        
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     19 27
US | Other post-retirement benefits        
Disclosure of defined benefit plans [line items]        
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     8 143
Employer contributions     £ 8 10
Weighted average duration of defined benefit obligation     11 years  
Restrictions on asset recognised £ 77   £ 244 77
US | Other post-retirement benefits | Plan assets        
Disclosure of defined benefit plans [line items]        
Decrease (increase) in net defined benefit liability (asset) resulting from resulting from contributions to plan by employer     £ 8 £ 143
Top of range | UK | National Grid UK Retirement Plan (NGUKRP)        
Disclosure of defined benefit plans [line items]        
Defined contribution plan, employer matching contribution percent of gross pay     12.00%  
Top of range | US        
Disclosure of defined benefit plans [line items]        
Defined contribution plan, employer core contribution as a percent of salary     9.00%  
Defined contribution plan, employer matching contribution, percent of match     50.00%  
Bottom of range | US        
Disclosure of defined benefit plans [line items]        
Defined contribution plan, employer matching contribution percent of gross pay     8.00%  
Defined contribution plan, employer core contribution as a percent of salary     3.00%  
Defined contribution plan, employer matching contribution, percent of match     25.00%