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Other (expense) income and finance costs - Finance costs (Details) - USD ($)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of other income & finance costs [Line Items]    
Capitalized interest paid $ 0 $ 3,848,000
Other interest and financing costs 4,397,000 3,196,000
Senior secured notes redemption premium 6,275,000 0
Amortization of discount and transaction costs of senior secured notes due to early redemption 2,286,000 0
Interest expense on lease liabilities 1,934,000 1,828,000
Asset retirement obligation accretion 1,945,000 2,532,000
Total finance costs 50,943,000 49,114,000
Less: Capitalized interest 0 3,848,000
Finance costs 50,943,000 45,266,000
Senior Secured Notes due 2024    
Disclosure of other income & finance costs [Line Items]    
Capitalized interest paid 29,486,000 18,087,000
Term loan    
Disclosure of other income & finance costs [Line Items]    
Capitalized interest paid 6,380,000 6,611,000
Senior notes    
Disclosure of other income & finance costs [Line Items]    
Capitalized interest paid 0 17,525,000
Senior Notes 1 and Revolving Credit Facility 1    
Disclosure of other income & finance costs [Line Items]    
Write-off of unamortized transaction costs of 2012 notes and ARCA (note 16(b)) 0 3,559,000
Redemption option derivative asset    
Disclosure of other income & finance costs [Line Items]    
Redemption option derivative gain (note 16(a)) $ (1,760,000) $ (4,224,000)