Financial instruments - Aggregate accounts receivable, net of allowance for doubtful accounts (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts receivable	$ 9,657	$ 10,255
Allowance for doubtful accounts receivable, current	3,900	3,500
Spain, Italy, Portugal, and Greece
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current and long-term trade accounts receivable in Greece, Italy, Spain and Portugal	$ 51,468	$ 49,054
Percentage of total net current and long-term trade accounts receivable - Greece, Italy, Spain and Portugal	14.10%	14.60%
Long-term portion of accounts receivable, net	$ 4,200	$ 3,300