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Concentrations of risk and geographic information - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Revenue, Major Customer [Line Items]      
Revenue $ 891,925 $ 1,194,651 $ 1,148,337
United States [Member]      
Revenue, Major Customer [Line Items]      
Revenue 428,300 429,900 $ 401,100
Outside the United States [Member]      
Revenue, Major Customer [Line Items]      
Long-lived assets $ 6,900 $ 11,000