Retirement Benefits (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2019	Jun. 29, 2018	Jun. 28, 2019	Jun. 29, 2018
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,635	$ 1,998	$ 3,644	$ 4,211
Interest cost	3,572	3,411	7,310	6,845
Expected return on assets	(4,216)	(4,632)	(8,575)	(8,718)
Amortization and other	1,838	2,080	3,817	4,175
Net periodic benefit cost	2,829	2,857	6,196	6,513
Postretirement Medical
Defined Benefit Plan Disclosure [Line Items]
Service cost	123	175	273	350
Interest cost	264	265	581	529
Amortization	29	136	137	272
Net periodic benefit cost	$ 416	$ 576	$ 991	$ 1,151