Income taxes (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 27, 2013	Dec. 28, 2012	Sep. 28, 2012	Sep. 27, 2013	Sep. 28, 2012
Income tax expense benefit and effective income tax rate [Abstract]
Income tax expense	$ 1,937,000		$ 356,000	$ 2,021,000	$ 2,504,000
Effective tax rate (in hundredths)				(11.30%)	(14.50%)
Increase (decrease) to prior period income tax expense		(800,000)
Accrued interest and /or penalties related to uncertain income tax position	500,000			500,000
Increase (decrease) in accrued interest and / or penalties related to uncertain income tax position				200,000
Total unrecognized tax benefits	3,600,000			3,600,000
Net decrease in unrecognized income tax benefits				4,300,000
Unrecognized tax benefits in other long-term liabilities	1,400,000			1,400,000
Unrecognized tax benefits recorded as an offset to deferred tax asets	2,200,000			2,200,000
Reclassification adjustment due to recent accounting guidance				1,700,000
Estimated unrecognized tax benefits to be resolved within next twelve months	$ 2,200,000			$ 2,200,000