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Concentrations of risk and geographic information - 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, Major Customer [Line Items]          
Revenue $ 134,246,000 $ 292,429,000 $ 253,646,000 $ 535,137,000  
Document Period End Date     Jun. 30, 2020    
United States [Member]          
Revenue, Major Customer [Line Items]          
Revenue 76,800,000 $ 117,700,000 $ 122,500,000 $ 200,700,000  
Outside the United States [Member]          
Revenue, Major Customer [Line Items]          
Long-lived assets $ 8,500,000   $ 8,500,000   $ 11,000,000.0