Shareholders' Equity - Changes in AOCI (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 25, 2021	Jun. 26, 2020	Jun. 25, 2021	Jun. 26, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Accumulated other comprehensive income (loss)	$ (131,999)	$ (172,330)	$ (124,165)	$ (169,787)
Other comprehensive income (loss) before reclassifications	5,693	6,756	(4,777)	2,079
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,838	1,892	4,474	4,026
Ending Accumulated other comprehensive income (loss)	(124,468)	(163,682)	(124,468)	(163,682)
Pension and Postretirement Medical
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Accumulated other comprehensive income (loss)	(111,493)	(111,587)	(114,129)	(113,721)
Other comprehensive income (loss) before reclassifications	0	0	0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	1,838	1,892	4,474	4,026
Ending Accumulated other comprehensive income (loss)	(109,655)	(109,695)	(109,655)	(109,695)
Cumulative Translation Adjustment
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Accumulated other comprehensive income (loss)	(20,506)	(60,743)	(10,036)	(56,066)
Other comprehensive income (loss) before reclassifications	5,693	6,756	(4,777)	2,079
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0	0	0	0
Ending Accumulated other comprehensive income (loss)	$ (14,813)	$ (53,987)	$ (14,813)	$ (53,987)

- Definition

Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.

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