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Post employment benefits - Weighted average assumptions used to determine group's deficit/surplus in main post employment plans (Detail)
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2017
United Kingdom      
Disclosure of defined benefit plans [line items]      
Rate of general increase in salaries 3.60% 4.30% 4.40%
Rate of increase to pensions in payment 3.20% 3.30% 3.40%
Rate of increase to deferred pensions 2.20% 2.10% 2.20%
Discount rate for plan liabilities 2.30% 2.80% 2.60%
United Kingdom | CPI      
Disclosure of defined benefit plans [line items]      
Inflation 2.20% 2.10% 2.20%
United Kingdom | RPI      
Disclosure of defined benefit plans [line items]      
Inflation 3.20% 3.10% 3.20%
Ireland      
Disclosure of defined benefit plans [line items]      
Rate of general increase in salaries 2.30% 3.20% 3.00%
Rate of increase to pensions in payment 1.50% 2.00% 1.70%
Rate of increase to deferred pensions 1.30% 1.80% 1.60%
Discount rate for plan liabilities 1.20% 1.70% 2.10%
Ireland | CPI      
Disclosure of defined benefit plans [line items]      
Inflation 1.30% 1.80% 1.60%
United States      
Disclosure of defined benefit plans [line items]      
Rate of general increase in salaries 0.00% 0.00% 0.00%
Rate of increase to pensions in payment 0.00% 0.00% 0.00%
Rate of increase to deferred pensions 0.00% 0.00% 0.00%
Discount rate for plan liabilities 3.40% 4.10% 3.70%
United States | CPI      
Disclosure of defined benefit plans [line items]      
Inflation 1.70% 2.10% 1.80%