Other Postretirement Benefits - Changes in the Benefit Obligation and Plan Assets and Other Amounts (Details) - Other Postretirement Benefit Plans [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 25, 2016	Dec. 27, 2015
Change in benefit obligation
Benefit obligation at beginning of year	$ 65,042	$ 71,047
Service cost	367	417	$ 588
Interest cost	1,881	1,979	2,794
Plan participants’ contributions	4,007	4,409
Actuarial loss	3,703	28
Curtailments/settlements	(10,328)	0
Benefits paid	(10,030)	(12,838)
Benefit obligation at end of year	54,642	65,042	71,047
Change in plan assets
Fair value of plan assets at beginning of year	0	0
Employer contributions	6,023	8,429
Plan participants’ contributions	4,007	4,409
Benefits paid	(10,030)	(12,838)
Fair value of plan assets at end of year	0	0	$ 0
Amount recognized in the Consolidated Balance Sheets
Current liabilities	(5,826)	(7,043)
Noncurrent liabilities	(48,816)	(57,999)
Net amount recognized	(54,642)	(65,042)
Amount recognized in accumulated other comprehensive loss
Actuarial loss	38,512	22,522
Prior service credit	(18,613)	(32,870)
Total	$ 19,899	$ (10,348)