Other Postretirement Benefits - Changes in the Benefit Obligation and Plan Assets and Other Amounts (Details) - Other Postretirement Benefit Plans, Defined Benefit [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 25, 2016	Dec. 27, 2015	Dec. 28, 2014
Change in benefit obligation
Benefit obligation at beginning of year	$ 71,047	$ 81,054
Service cost	417	588	$ 580
Interest cost	1,979	2,794	3,722
Plan participants’ contributions	4,409	4,230
Actuarial (gain)/loss	28	(5,543)
Plan amendments	0	1,145
Benefits paid	(12,838)	(13,221)
Benefit obligation at end of year	65,042	71,047	81,054
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Employer contributions	8,429	8,991
Plan participants’ contributions	4,409	4,230
Benefits paid	(12,838)	(13,221)
Fair value of plan assets at end of year	0	0	$ 0
Amount recognized in the Consolidated Balance Sheets
Current liabilities	(7,043)	(8,168)
Noncurrent liabilities	(57,999)	(62,879)
Net amount recognized	(65,042)	(71,047)
Amount recognized in accumulated other comprehensive loss
Actuarial loss	22,522	26,599
Prior service cost	(32,870)	(41,309)
Total amount recognized in accumulated other comprehensive loss	$ (10,348)	$ (14,710)