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Summary of significant accounting policies - Narrative (Details) - USD ($)
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]          
Allowance for credit loss $ 0   $ 0    
Inventory write-down   $ 0 200,000 $ 0  
Letters of Credit 7,500,000   $ 7,500,000    
Term for letter of credit (in years)     5 years    
Operating lease ROU asset 29,475,000   $ 29,475,000   $ 13,693,000
Allowance for doubtful accounts receivable 200,000   200,000   300,000
Deferred tax liability         $ 122,000,000.0
Accounts receivable, allowance for credit loss, 0   0    
Fifth Anniversary          
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]          
Letters of Credit $ 3,800,000   $ 3,800,000