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Post-retirement benefits - Summary of fair value of plan assets (Detail)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of information about defined benefit plans [abstract]    
Equities 21.10% 20.40%
– Quoted 17.90% 17.00%
– Private 3.20% 3.40%
Bonds 55.80% 63.40%
– Government fixed income 17.70% 18.40%
– Government inflation-linked 9.60% 16.20%
– Corporate and other publicly quoted 22.60% 24.10%
– Private 5.90% 4.70%
Property 7.50% 8.80%
– Quoted property funds 3.10% 3.40%
– Unquoted property funds 4.40% 5.40%
Qualifying insurance policies 11.30% 3.10%
Cash & other 4.30% 4.30%
Total 100.00% 100.00%