Severance, impairment and other associated costs (Details) (USD $)	6 Months Ended
	Jun. 27, 2014	Jun. 28, 2013
Restructuring Reserve [Roll Forward]
Balance accrued at beginning of period	$ 493,000
Net expense	1,937,000
Cash payments	(631,000)
Non-cash charges	0
Balance accrued at end of period	1,799,000
Restructuring and impairment charges	1,900,000	100,000
Second Quarter 2013 program [Member]
Restructuring Reserve [Roll Forward]
Severance and related costs	200,000
Third Quarter 2013 program [Member]
Restructuring Reserve [Roll Forward]
Severance and related costs	$ 1,700,000