Shareholders' Equity - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 26, 2020	Jun. 28, 2019	Jun. 26, 2020	Jun. 28, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Accumulated other comprehensive income (loss)	$ (172,330)	$ (142,031)	$ (169,787)	$ (144,857)
Other comprehensive income (loss) before reclassifications	6,756	1,232	2,079	2,401
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,892	1,487	4,026	3,144
Ending Accumulated other comprehensive income (loss)	(163,682)	(139,312)	(163,682)	(139,312)
Pension and Postretirement Medical
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Accumulated other comprehensive income (loss)	(111,587)	(85,232)	(113,721)	(86,889)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,892	1,487	4,026	3,144
Ending Accumulated other comprehensive income (loss)	(109,695)	(83,745)	(109,695)	(83,745)
Cumulative Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Accumulated other comprehensive income (loss)	(60,743)	(56,799)	(56,066)	(57,968)
Other comprehensive income (loss) before reclassifications	6,756	1,232	2,079	2,401
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0	0	0
Ending Accumulated other comprehensive income (loss)	$ (53,987)	$ (55,567)	$ (53,987)	$ (55,567)