Changes in shareholders' equity Accumulated other comprehensive income into income expense (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 29, 2015		Mar. 30, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold	$ 206,793		$ 217,387
Income from continuing operations before taxes	48,605		43,803
Tax benefit (expense)	(9,332)		(8,534)
Income from continuing operations	39,273		35,269
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations	897		740
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	(209)		(77)
Tax benefit (expense)	10		34
Income from continuing operations	(199)		(43)
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	1,606		1,097
Tax benefit (expense)	(510)		(314)
Income from continuing operations	1,096		783
Actuarial losses	1,606	[1]	1,102	[1]
Prior-service costs	0	[1]	(5)	[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	$ (209)		$ (77)

[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).