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Concentrations of risk and geographic information - Narrative (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Dec. 31, 2015
Revenue, Major Customer [Line Items]          
Revenue $ 240,569 $ 400,340 $ 644,860 $ 1,183,368  
United States [Member]          
Revenue, Major Customer [Line Items]          
Revenue 122,500 $ 169,200 310,700 $ 513,600  
Outside the United States [Member]          
Revenue, Major Customer [Line Items]          
Long-lived assets $ 71,200   $ 71,200   $ 47,600