Retirement Benefits (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jul. 13, 2018	Sep. 28, 2018	Sep. 29, 2017	Sep. 28, 2018	Sep. 29, 2017
Defined Benefit Plan Disclosure [Line Items]
Increase in operating earnings and non-operating expense				$ (15,500)
Defined Benefit Plan, Contributions by Employer	$ 40,000
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 2,165	$ 1,917	6,376	$ 5,732
Interest cost		3,227	3,874	10,072	11,477
Expected return on assets		(4,369)	(4,236)	(13,087)	(12,700)
Amortization and other		2,077	2,408	6,252	6,932
Net periodic benefit cost		3,100	3,963	9,613	11,441
Postretirement Medical
Defined Benefit Plan Disclosure [Line Items]
Service cost		127	150	477	451
Interest cost		284	274	813	820
Amortization		213	(2)	485	(7)
Net periodic benefit cost		$ 624	422	$ 1,775	1,264
Accounting Standards Update 2017-07 [Member]
Defined Benefit Plan Disclosure [Line Items]
Increase in operating earnings and non-operating expense			$ 1,800		$ 5,100