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Income taxes - Summary of unrecognized tax losses (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
2025-2040 | Canada    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount of the tax losses $ 512,102  
2025-2039 | Canada    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount of the tax losses   $ 487,229
Capital losses with no expiry | Canada    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount of the tax losses 65,836 63,483
2021-2025 | Greece    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount of the tax losses 140,196  
2020-2024 | Greece    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount of the tax losses   98,395
No Expiry | Brazil    
Disclosure of temporary difference, unused tax losses and unused tax credits [line items]    
Gross amount of the tax losses $ 2,421 $ 31,128