Retirement Benefits (Components of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2019	Dec. 28, 2018	Dec. 29, 2017
Components of net periodic benefit cost
Settlement loss (gain)			$ 12,000
Pension Benefits
Components of net periodic benefit cost
Service cost-benefits earned during the period	$ 7,735	$ 8,487	7,675
Interest cost on projected benefit obligation	15,103	13,424	15,044
Expected return on assets	(17,152)	(17,447)	(17,186)
Amortization of prior service cost (credit)	279	279	255
Amortization of net loss (gain)	8,392	7,931	8,634
Settlement loss (gain)	0	184	12,313
Cost of pension plans which are not significant and have not adopted ASC 715	110	106	122
Net periodic benefit cost	14,467	12,964	26,857
Postretirement Medical Benefits
Components of net periodic benefit cost
Service cost-benefits earned during the period	545	636	601
Interest cost on projected benefit obligation	1,162	1,084	1,093
Expected return on assets	0	0	0
Amortization of prior service cost (credit)	0	0	(344)
Amortization of net loss (gain)	273	646	334
Settlement loss (gain)	0	0	0
Net periodic benefit cost	1,980	2,366	1,684
Accounting Standards Update 2017-07 [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-service pension cost included in non-operating expense	$ 5,000	$ 8,000	$ 18,000