Changes in shareholders' equity Accumulated other comprehensive income into income expense (Details) - Reclassification out of Accumulated Other Comprehensive Income - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 28, 2015	Jun. 29, 2014	Jun. 28, 2015	Jun. 29, 2014
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold		$ 218,808	$ 224,017	$ 425,601	$ 441,404
Income from continuing operations before taxes		50,479	58,836	99,084	102,639
Tax benefit (expense)		(5,280)	(10,006)	(14,612)	(18,540)
Income from continuing operations		45,199	48,830	84,472	84,099
Income from continuing operations		1,403	668	2,300	1,408
Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations before taxes		472	(52)	263	(129)
Tax benefit (expense)		(110)	5	(100)	39
Income from continuing operations		362	(47)	163	(90)
Pension and Other Postretirement Benefits Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Income from continuing operations before taxes		1,605	1,097	3,211	2,194
Tax benefit (expense)		(564)	(382)	(1,074)	(696)
Income from continuing operations		1,041	715	2,137	1,498
Actuarial losses	[1]	1,605	1,103	3,211	2,205
Prior-service costs	[1]	0	(6)	0	(11)
Commodity Contract | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Cost of goods sold		$ 472	$ (52)	$ 263	$ (129)

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