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Loss per share - Narrative (Details) - $ / shares
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
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   9,277,305    
Stock Compensation Plan          
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]          
Shares excluded from calculation (in shares) 963,171 295,526 741,120 285,975  
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 9,277,305 4,247,850  
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