Other Postretirement Benefits - Changes in the Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 25, 2016	Dec. 27, 2015	Dec. 28, 2014
Other Postretirement Benefits
Additional postretirement costs	$ 15,000	$ 16,000	$ 18,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Other Postretirement Benefits
Net actuarial (gain)/loss	28	(5,543)	8,882
Prior service cost/(credit)	0	1,145	(25,489)
Amortization of loss	(4,105)	(5,197)	(4,948)
Amortization of prior service credit	8,440	9,495	7,199
Total recognized in other comprehensive (income)/loss	4,363	(100)	(14,356)
Net periodic postretirement benefit expense/(income)	(1,939)	(916)	4,402
Total recognized in net periodic benefit cost and other comprehensive loss	2,424	$ (1,016)	$ (9,954)
Estimated actuarial loss that will be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year	3,000
Estimated prior service cost that will be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year	$ 8,000