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Loss per share - Narrative (Details) - $ / shares
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
May 28, 2019
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Maximum number of shares issuable if debt was converted (in shares) 9,277,305    
Stock Compensation Plan      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Shares excluded from calculation (in shares) 840,168 443,168  
Convertible Debt Securities      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Shares excluded from calculation (in shares) 9,277,305 9,277,305  
Convertible Senior Notes | 4% Convertible Senior Dotes due 2024      
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]      
Initial conversion price (in dollars per share) $ 22.3125   $ 22.3125