Other Postretirement Benefits - Changes in the Benefit Obligations Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 27, 2015	Dec. 28, 2014	Dec. 29, 2013
Other Postretirement Benefits
Additional postretirement costs	$ 16,000	$ 18,000	$ 20,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Other Postretirement Benefits
Net actuarial (gain)/loss	(5,543)	8,882	(13,500)
Prior service cost/(credit)	1,145	(25,489)	(1,690)
Amortization of loss	(5,197)	(4,948)	(4,440)
Amortization of prior service credit	9,495	7,199	13,051
Recognition of prior service credit due to curtailment	0	0	49,122
Total recognized in other comprehensive (income)/loss	(100)	(14,356)	42,543
Net periodic postretirement benefit expense/(income)	(916)	4,402	(52,543)
Total recognized in net periodic benefit cost and other comprehensive loss	(1,016)	$ (9,954)	$ (10,000)
Estimated actuarial loss that will be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year	4,100
Estimated prior service cost that will be amortized from accumulated other comprehensive loss into net periodic pension cost over the next fiscal year	$ 8,400