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Net income (loss) per common share - Reconciliation of numerator used for calculation of diluted net income per common share (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Earnings per share [abstract]    
Numerator for basic net income (loss) per common share $ (156,678) $ 87,767
Adjustment for the effect of TSARs:    
Cash-settled recovery included in net income 0 (5,433)
Equity-settled expense 0 (4,807)
Numerator for diluted net income (loss) per common share $ (156,678) $ 77,527