Accumulated Other Comprehensive Items - Reclassification Out of AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 26, 2020	Sep. 28, 2019	Sep. 26, 2020	Sep. 28, 2019
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Other expense, net	$ (32)	$ (98)	$ (95)	$ (296)
Interest expense	(1,670)	(3,066)	(6,060)	(10,143)
Cost of revenues	(86,294)	(99,257)	(263,510)	(302,852)
Total expense before income taxes	19,685	21,432	53,096	55,989
Income tax benefit	(4,705)	(5,219)	(13,738)	(12,310)
Net Income	14,980	16,213	39,358	43,679
Reclassification out of Accumulated Other Comprehensive Items
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Net Income	(59)	(20)	(61)	(113)
Reclassification out of Accumulated Other Comprehensive Items | Pension and Other Post-retirement Plans [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Total expense before income taxes	(16)	(16)	(48)	(48)
Income tax benefit	4	4	132	12
Net Income	(12)	(12)	84	(36)
Reclassification out of Accumulated Other Comprehensive Items | Recognized net actuarial loss
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Other expense, net	(14)	(16)	(43)	(48)
Reclassification out of Accumulated Other Comprehensive Items | Amortization of prior service cost
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Other expense, net	(2)	0	(5)	0
Reclassification out of Accumulated Other Comprehensive Items | Cash Flow Hedges
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Total expense before income taxes	(62)	(10)	(191)	(112)
Income tax benefit	15	2	46	35
Net Income	(47)	(8)	(145)	(77)
Reclassification out of Accumulated Other Comprehensive Items | Cash Flow Hedges | Interest rate swap agreements
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Interest expense	(109)	(10)	(215)	17
Reclassification out of Accumulated Other Comprehensive Items | Cash Flow Hedges | Forward currency-exchange contracts
Reclassification Adjustment out of Accumulated Other Comprehensive Items [Line Items]
Cost of revenues	$ 47	$ 0	$ 24	$ (129)