Income taxes (Details) (USD $)	3 Months Ended
	Mar. 28, 2014	Mar. 29, 2013
Income tax expense benefit and effective income tax rate [Abstract]
Income tax expense	$ 472,000	$ 434,000
Effective tax rate (in hundredths)	(5.50%)	(6.50%)
Accrued interest and penalties related to uncertain income tax positions	400,000
Increase (decrease) in accrued interest and penalties related to uncertain income tax positions	(100,000)
Total unrecognized tax benefits	3,600,000
Estimated unrecognized tax benefits to be resolved within next twelve months	1,800,000
Expected tax benefit	$ 100,000