v3.24.1.1.u2
Pensions and other post-retirement benefits - Narrative (Details)
£ in Millions
 12 Months Ended
Mar. 31, 2024
GBP (£)
plan
Mar. 31, 2023
GBP (£)
Mar. 31, 2022
GBP (£)
Disclosure of defined benefit plans [line items]      
Independent actuarial funding valuation, period 3 years    
Number of plans showing funding shortfall | plan 3    
Recovery plan payment amount     £ 100
Employer contributions £ 165 £ 284  
Plan assets      
Disclosure of defined benefit plans [line items]      
Employer contributions 165 284  
UK | Pensions      
Disclosure of defined benefit plans [line items]      
Recovery plan payments to be paid 12    
Employer contributions, excluding recovery plan payments 95 74  
Employer contributions £ 118 197  
Weighted average duration of defined benefit obligation 11 years    
UK | Pensions | Maximum      
Disclosure of defined benefit plans [line items]      
Value of security arrangements £ 180    
UK | Pensions | Plan assets      
Disclosure of defined benefit plans [line items]      
Employer contributions £ 118 197  
US      
Disclosure of defined benefit plans [line items]      
Defined contribution plan, employer matching contribution percent of gross pay 8.00%    
US | Pensions      
Disclosure of defined benefit plans [line items]      
Number of defined benefit plans | plan 4    
Employer contributions £ 26 76  
Weighted average duration of defined benefit obligation 11 years    
US | Pensions | Plan assets      
Disclosure of defined benefit plans [line items]      
Employer contributions £ 26 76  
US | Other post-retirement benefits      
Disclosure of defined benefit plans [line items]      
Employer contributions £ 21 11  
Weighted average duration of defined benefit obligation 12 years    
US | Other post-retirement benefits | Plan assets      
Disclosure of defined benefit plans [line items]      
Employer contributions £ 21 £ 11  
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 core contribution as a percent of salary 3.00%    
Defined contribution plan, employer matching contribution, percent of match 25.00%