Financial instruments - Aggregate accounts receivable, net of allowance for doubtful accounts (Details) - USD ($) $ in Thousands	1 Months Ended
	Jan. 31, 2017	Oct. 01, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts receivable		$ 10,084	$ 8,630
Allowance for doubtful accounts		3,100	2,000
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		$ 50,613	$ 51,098
Percentage of total net current and long-term trade accounts receivable - Greece, Italy, Spain and Portugal		16.90%	19.30%
Long-term portion of accounts receivable, net		$ 3,500	$ 2,700
Italy
Accounts, Notes, Loans and Financing Receivable [Line Items]
Financing receivable, significant sales	$ 16,100
Proceeds from sale of receivables	$ 16,000