Retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 27, 2019	Sep. 28, 2018	Sep. 27, 2019	Sep. 28, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 2,161	$ 2,165	$ 5,805	$ 6,376
Interest cost	3,711	3,227	11,021	10,072
Expected return on assets	(4,289)	(4,369)	(12,864)	(13,087)
Amortization and other	3,095	2,077	6,912	6,252
Net periodic benefit cost	4,678	3,100	10,874	9,613
Postretirement Medical
Defined Benefit Plan Disclosure [Line Items]
Service cost	136	127	409	477
Interest cost	291	284	872	813
Amortization	68	213	205	485
Net periodic benefit cost	$ 495	$ 624	$ 1,486	$ 1,775