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EMPLOYEE BENEFITS - Obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 27, 2015
Dec. 28, 2014
Dec. 29, 2013
Change in Benefit Obligation      
Net retirement expenses $ 9,971 $ 4,632 $ 12,162
Pension plan      
Change in Benefit Obligation      
Service cost 11,680 8,030 5,545
Interest cost 84,994 91,004 84,596
Expected return on plan assets (106,283) (107,460) (101,053)
Prior service cost amortization   12 14
Actuarial loss 22,194 16,009 25,557
Net pension expense 12,585 7,595 14,659
Post-retirement plans      
Change in Benefit Obligation      
Interest cost 368 514  
Net pension expense $ (2,614) $ (2,963) $ (2,497)