Pension Benefits - Net Periodic Pension Cost (Details) - USD ($) $ in Thousands	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Sep. 24, 2017	Jun. 25, 2017	Mar. 26, 2017	Dec. 25, 2016	Sep. 25, 2016	Jun. 26, 2016	Mar. 27, 2016	Mar. 29, 2015	Dec. 31, 2017	Dec. 25, 2016	Dec. 27, 2015
Pension Benefits
Pension settlement expense	$ 102,109	$ 0	$ 0	$ 0	$ 21,294	$ 0	$ 0	$ 0		$ 102,109	$ 21,294	$ 40,329
Pension Plan [Member]
Pension Benefits
Service cost										9,799	9,134	12,089
Interest cost										68,582	74,465	84,596
Expected return on plan assets										(102,900)	(111,159)	(115,261)
Amortization and other costs										33,369	32,458	41,523
Amortization of prior service (credit)/cost										(1,945)	(1,945)	(1,945)
Effect of settlement/curtailment										102,109	19,695	40,329
Net periodic pension cost										109,014	22,648	61,331
Reduction in pension benefit obligation	263,300				52,200				$ 142,800
Pension settlement expense	102,100				21,300				40,300
Discretionary contributions to pension plans	$ 120,000
Lump-sum payments made from Company cash					49,500				$ 98,300
Curtailment					$ 1,600
Qualified Plan [Member] | Pension Plan [Member]
Pension Benefits
Service cost										9,720	8,991	11,932
Interest cost										60,742	66,293	74,536
Expected return on plan assets										(102,900)	(111,159)	(115,261)
Amortization and other costs										29,051	28,274	36,442
Amortization of prior service (credit)/cost										(1,945)	(1,945)	(1,945)
Effect of settlement/curtailment										102,109	21,294	40,329
Net periodic pension cost										96,777	11,748	46,033
Pension contributions										127,600
Nonqualified Plan [Member]
Pension Benefits
Effect of settlement/curtailment										0
Nonqualified Plan [Member] | Pension Plan [Member]
Pension Benefits
Service cost										79	143	157
Interest cost										7,840	8,172	10,060
Expected return on plan assets										0	0	0
Amortization and other costs										4,318	4,184	5,081
Amortization of prior service (credit)/cost										0	0	0
Effect of settlement/curtailment											(1,599)	0
Net periodic pension cost										$ 12,237	$ 10,900	$ 15,298