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Financial risk management, objectives and policies - Maximum exposure to credit risk (Details) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Disclosure of credit risk exposure [abstract]    
Cash and cash equivalents $ 94,944 $ 77,292
Accounts receivable and accrued revenues 60,196 42,212
Maximum credit exposure $ 155,140 $ 119,504