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Concentrations of risk and geographic information - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Revenue, Major Customer [Line Items]          
Revenue $ 131,169 $ 285,936 $ 666,306 $ 770,959  
Document Period End Date     Sep. 30, 2019    
United States [Member]          
Revenue, Major Customer [Line Items]          
Revenue 50,900 $ 93,200 $ 251,600 $ 264,900  
Outside the United States [Member]          
Revenue, Major Customer [Line Items]          
Long-lived assets $ 13,300   $ 13,300   $ 15,900