Financial instruments - Aggregate accounts receivable, net of allowance for doubtful accounts (Details) - USD ($) $ in Thousands	Sep. 25, 2016	Dec. 31, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts receivable	$ 9,000	$ 8,000
Spain, Italy, Portugal, and Greece
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for doubtful accounts receivable	8,000	7,200
Current and long-term trade accounts receivable (net of allowances of $8.0 million and $7.2 million at September 25, 2016 and December 31, 2015, respectively) in Greece, Italy, Spain and Portugal (1)	$ 59,785	$ 62,272
Percentage of total net current and long-term trade accounts receivable - Greece, Italy, Spain and Portugal	22.90%	23.90%
Long-term portion of accounts receivable, net	$ 7,700	$ 8,100