Income taxes (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 27, 2014	Jun. 28, 2013	Jun. 27, 2014	Jun. 28, 2013
Income tax expense benefit and effective income tax rate [Abstract]
Income tax expense	$ 3,520,000	$ (349,000)	$ 3,992,000	$ 84,000
Effective tax rate (in hundredths)			(31.00%)	(0.70%)
Accrued interest and penalties related to uncertain income tax positions	400,000		400,000
Total unrecognized tax benefits	3,600,000		3,600,000
Net increase in unrecognized income tax benefits			100,000
Estimated unrecognized tax benefits to be resolved within next twelve months	1,700,000		1,700,000
Increase (decrease) in accrued interest and penalties related to uncertain income tax positions			$ (100,000)