Changes in shareholders' equity Accumulated other comprehensive income into income expense (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 28, 2014		Sep. 29, 2013		Sep. 28, 2014		Sep. 29, 2013
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 221,007		$ 209,804		$ 662,411		$ 631,730
Income from continuing operations before taxes	64,912		50,988		167,551		135,839
Tax benefit	(9,684)		(5,209)		(28,224)		(18,958)
Income from continuing operations	55,228		45,779		139,327		116,881
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations	359		1,781		1,767		3,815
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	(397)		850		(526)		154
Tax benefit	49		(158)		88		164
Income from continuing operations	(348)		692		(438)		318
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	1,085		1,693		3,279		5,449
Tax benefit	(378)		(604)		(1,074)		(1,952)
Income from continuing operations	707		1,089		2,205		3,497
Actuarial losses/(gains)	1,090	[1]	1,696	[1]	3,295	[1]	5,460	[1]
Prior-service costs	(5)	[1]	(4)	[1]	(16)	[1]	(15)	[1]
Transition obligation	0	[1]	1	[1]	0	[1]	4	[1]
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	$ (397)		$ 850		$ (526)		$ 154

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