EMPLOYEE BENEFITS - Fair value (Details) - USD ($) $ in Thousands	Dec. 27, 2015	Dec. 28, 2014	Dec. 29, 2013
EMPLOYEE BENEFITS
Pending trades	$ 9,003
Fair value of plan assets	1,349,603
Pension plan
EMPLOYEE BENEFITS
Total	1,340,600	$ 1,478,686
Fair value of plan assets	1,349,603	1,478,686	$ 1,432,695
Cash and cash equivalents | Pension plan
EMPLOYEE BENEFITS
Total	844	1,068
Mutual funds | Pension plan
EMPLOYEE BENEFITS
Total	436,316	485,488
Corporate debt instruments | Pension plan
EMPLOYEE BENEFITS
Total	112	106
Common collective trust | Pension plan
EMPLOYEE BENEFITS
Total	845,686	937,809
Real Estate | Pension plan
EMPLOYEE BENEFITS
Total	50,360	47,579
Other | Pension plan
EMPLOYEE BENEFITS
Total	7,282	6,636
Level 1 | Pension plan
EMPLOYEE BENEFITS
Total	437,160	486,556
Level 1 | Cash and cash equivalents | Pension plan
EMPLOYEE BENEFITS
Total	844	1,068
Level 1 | Mutual funds | Pension plan
EMPLOYEE BENEFITS
Total	436,316	485,488
Level 2 | Pension plan
EMPLOYEE BENEFITS
Total	845,798	937,915
Level 2 | Corporate debt instruments | Pension plan
EMPLOYEE BENEFITS
Total	112	106
Level 2 | Common collective trust | Pension plan
EMPLOYEE BENEFITS
Total	845,686	937,809
Level 3 | Pension plan
EMPLOYEE BENEFITS
Total	57,642	54,215	$ 59,432
Level 3 | Real Estate | Pension plan
EMPLOYEE BENEFITS
Total	50,360	47,579
Level 3 | Other | Pension plan
EMPLOYEE BENEFITS
Total	$ 7,282	$ 6,636