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Basis of preparation - Narrative (Details)
€ in Millions, £ in Millions
6 Months Ended
Dec. 31, 2021
€ / £
$ / £
Dec. 31, 2020
EUR (€)
$ / £
€ / £
Dec. 31, 2020
GBP (£)
$ / £
€ / £
Euro      
Disclosure of foreign exchange rates [Line Items]      
Weighted average exchange rate used in the translation of income statements | € / £ 1.17 1.11  
Exchange rates used to translate assets and liabilities | € / £ 1.19 1.11 1.11
Euro | European Debt Issuance Programme      
Disclosure of foreign exchange rates [Line Items]      
Notional amount bonds | €   € 700  
Pound Sterling | European Debt Issuance Programme      
Disclosure of foreign exchange rates [Line Items]      
Notional amount bonds | £     £ 400
US Dollar      
Disclosure of foreign exchange rates [Line Items]      
Weighted average exchange rate used in the translation of income statements | $ / £ 1.36 1.31  
Exchange rates used to translate assets and liabilities | $ / £ 1.35 1.36 1.36