Changes in shareholders' equity Accumulated other comprehensive income into income expense (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 27, 2015	Sep. 28, 2014	Sep. 27, 2015	Sep. 28, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold		$ 215,501	$ 221,007	$ 641,102	$ 662,411
Income from continuing operations before taxes		62,374	64,912	161,458	167,551
Tax benefit (expense)		(803)	(9,684)	(15,415)	(28,224)
Income from continuing operations		61,571	55,228	146,043	139,327
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations		1,967	359	4,267	1,767
Reclassification out of Accumulated Other Comprehensive Income | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations before taxes		1,168	(397)	1,431	(526)
Tax benefit (expense)		(221)	49	(321)	88
Income from continuing operations		947	(348)	1,110	(438)
Reclassification out of Accumulated Other Comprehensive Income | Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations before taxes		1,571	1,085	4,782	3,279
Tax benefit (expense)		(551)	(378)	(1,625)	(1,074)
Income from continuing operations		1,020	707	3,157	2,205
Actuarial losses	[1]	1,571	1,090	4,782	3,295
Prior-service costs	[1]	0	(5)	0	(16)
Reclassification out of Accumulated Other Comprehensive Income | Commodity Contract | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold		$ 1,168	$ (397)	$ 1,431	$ (526)

[1] These accumulated other comprehensive (loss) income components are included in the computation of net benefit cost of pension and other postretirement benefit plans (see Note 12 for additional information).