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Related parties - Summary of Remuneration to Related Parties (Parenthetical) (Detail) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
N Holland [Member]      
Disclosure of transactions between related parties [line items]      
Salary paid, executive directors and prescribed officers $ 0 $ 406,700  
P Schmidt [Member]      
Disclosure of transactions between related parties [line items]      
Salary paid, executive directors and prescribed officers 0 124,150  
B Mattison [Member]      
Disclosure of transactions between related parties [line items]      
Salary paid, executive directors and prescribed officers 0   $ 88,217
S Reid [Member] | Non Executive Directors [member]      
Disclosure of transactions between related parties [line items]      
Services received from related party 14,351,000 0  
A Andani [Member] | Non Executive Directors [member]      
Disclosure of transactions between related parties [line items]      
Services received from related party $ 70,021,000 $ 141,605,000