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Income taxes - Effective income tax rate (Detail)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Income taxes [Abstract]      
Weighted average statutory income tax rate 25.00% 25.20% 24.90%
Recognition of previously unrecognized tax loss and credit carryforwards rate (0.40%) (3.90%) (0.40%)
Unrecognized tax loss and credit carryforwards rate 0.40% 0.10% 0.50%
Changes to Recognition of Temporary Differences Rate (1.30%) 2.10% (0.30%)
Tax rate effect of revenues exempt from taxation (10.80%) (9.50%) (11.90%)
Tax rate effect of expense not deductible in determining taxable profit (tax loss) 5.80% 5.30% 3.70%
Withholding and other taxes 0.50% 3.70% 4.50%
Tax rate effect from change in tax rate (0.90%) (0.10%) 1.80%
Tax rate effect of adjustments for current tax of prior periods (0.10%) 0.60% (1.30%)
Tax expenses (benefit) due to other tax liabilities 0.90% (1.60%) (8.60%)
Other tax rate effects for reconciliation between accounting profit and tax expense (income) (0.10%) 0.20% (0.10%)
Average effective tax rate 19.00% 22.10% 12.80%