Pension and Other Postretirement Benefits - Other Postretirement Benefits (Details) - Other Postretirement Benefit Plan - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 27, 2021	Jun. 28, 2020	Jun. 27, 2021	Jun. 28, 2020
Pension Benefits
Service cost	$ 13	$ 7	$ 26	$ 14
Interest cost	141	257	282	513
Amortization of actuarial loss	852	763	1,704	1,526
Amortization of prior service credit	(836)	(1,223)	(1,672)	(2,279)
Net periodic postretirement benefit cost/(income)	$ 170	$ (196)	$ 340	$ (226)