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Summary of significant accounting policies - Narrative (Details) - USD ($)
$ in Thousands
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jan. 01, 2019
Dec. 31, 2018
Jan. 01, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]          
Operating lease ROU asset $ 10,387   $ 11,300    
Allowance for doubtful accounts receivable 500     $ 700  
Deferred tax liability 122,032     122,032  
Retained earnings (accumulated deficit) (1,052,825)     $ (938,923)  
ASC 606 Adjustment $ 0 $ (3,937)      
Adjustments          
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]          
Retained earnings (accumulated deficit)         $ 3,300