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Fair value measurements (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Nov. 24, 2020
Dec. 31, 2019
Apr. 12, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash $ 308,200   $ 145,900  
Marketable securities 0   14,847  
Restricted Cash 2,000   0  
Fair Value, Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents 19,445   4,413  
Marketable securities 0   14,847  
Fair Value, Recurring [Member] | Money Market Funds [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents 19,445   4,413  
Fair Value, Recurring [Member] | Level 1 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents 19,445   4,413  
Marketable securities 0   0  
Fair Value, Recurring [Member] | Level 1 [Member] | Money Market Funds [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents 19,445   4,413  
Fair Value, Recurring [Member] | Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents 0   0  
Marketable securities 0   14,847  
Fair Value, Recurring [Member] | Level 2 [Member] | Money Market Funds [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Cash and Cash Equivalents 0   0  
Convertible Senior Notes due 2022 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument       $ 175,000
Convertible Senior Notes due 2022 [Member] | Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value of convertible senior notes 146,000   170,000  
Convertible Senior Notes due 2025 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Debt Instrument   $ 143,800    
Convertible Senior Notes due 2025 [Member] | Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Fair value of convertible senior notes 166,800      
Corporate Debt Securities [Member] | Fair Value, Recurring [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Marketable securities 0   14,847  
Corporate Debt Securities [Member] | Fair Value, Recurring [Member] | Level 1 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Marketable securities 0   0  
Corporate Debt Securities [Member] | Fair Value, Recurring [Member] | Level 2 [Member]        
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]        
Marketable securities $ 0   $ 14,847