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Significant accounting policies - Text Details (Detail)
Dec. 31, 2019
Dec. 31, 2018
Bottom of range [member]    
Significant accounting policies - Text Details (Detail) [Line Items]    
Presumed significant influence, voting rights 20.00% 20.00%
Top of range [member]    
Significant accounting policies - Text Details (Detail) [Line Items]    
Presumed significant influence, voting rights 50.00% 50.00%