Financial instruments Aggregate accounts receivable, net of allowance for doubtful accounts (Details) - USD ($) $ in Thousands		6 Months Ended	12 Months Ended
		Jun. 28, 2015	Dec. 31, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current and long term accounts receivable, allowances		$ 2,700	$ 2,400
Spain, Italy, Portugal, and Greece
Accounts, Notes, Loans and Financing Receivable [Line Items]
Current and long-term accounts receivable (net of allowances of $7.7 million and $8.1 million at June 28, 2015 and December 31, 2014, respectively) in Greece, Italy, Spain and Portugal (1)	[1]	$ 76,384	$ 76,190
Percentage of total net current and long-term accounts receivable - Greece, Italy, Spain and Portugal		26.50%	27.30%
Current and long term accounts receivable, allowances		$ 7,700	$ 8,100
Long-term portion of accounts receivable, net		$ 9,100	$ 11,300

[1] June 28, 2015 and December 31, 2014 was $9.1 million and