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Details of treasury and other financial risks - Credit risk with number of counterparties (Detail)
Dec. 31, 2020
10-100 million [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 3
100-500 million [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 6
500 million and above [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 2
A+ rated [Member] | 10-100 million [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 2
A+ rated [Member] | 100-500 million [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 3
A+ rated [Member] | 500 million and above [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 1
A rated [Member] | 10-100 million [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 1
A rated [Member] | 500 million and above [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 1
A- rated [Member] | 100-500 million [Member]  
Details of treasury and other financial risks [Line Items]  
Counterparties 3