Other Postretirement Benefits - Changes in the Benefit Obligations Recognized in Other Comprehensive Income (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 25, 2016	Dec. 27, 2015
Other Postretirement Benefits
Net actuarial loss/(gain)	$ (6,625)	$ 28	$ (5,543)
Prior service cost	0	0	1,145
Amortization of loss	(3,621)	(4,105)	(5,197)
Amortization of prior service credit	7,755	8,440	9,495
Effect of curtailment	6,502	0	0
Effect of settlement	(26,235)	0	0
Total recognized in other comprehensive (income)/loss	30,246	4,363	(100)
Net periodic postretirement benefit income	(34,623)	(1,939)	(916)
Total recognized in net periodic postretirement benefit income and other comprehensive (income)/loss	(4,377)	2,424	(1,016)
Estimated actuarial loss that will be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year	5,000
Estimated prior service cost that will be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year	6,000
Additional postretirement costs	$ 15,000	$ 15,000	$ 16,000