Income taxes (Details) (USD $)	3 Months Ended
	Mar. 29, 2013	Mar. 30, 2012
Income tax expense benefit and effective income tax rate [Abstract]
Income tax expense	$ 434,000	$ 657,000
Effective tax rate (in hundredths)	(6.00%)	(18.00%)
Accrued interest and /or penalties related to uncertain income tax position	400,000
Increase (decrease) in accrued interest and / or penalties related to uncertain income tax position	100,000
Total unrecognized tax benefits	7,900,000
Unrecognized tax benefits in other long-term liabilities	$ 4,000,000