PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Fair Value Assumptions of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 27, 2020	Dec. 29, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	$ 305,983	$ 294,589
Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	1,487	11,582
Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	304,496	283,007
Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	1,487	11,582
Cash and cash equivalents | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	1,487	11,582
Cash and cash equivalents | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | Large U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	3,100	3,071
Union plan | Large U.S. equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | Large U.S. equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	3,100	3,071
Union plan | Large U.S. equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | Small/Mid U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	392	372
Union plan | Small/Mid U.S. equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | Small/Mid U.S. equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	392	372
Union plan | Small/Mid U.S. equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	1,874	1,878
Union plan | International equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | International equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	1,874	1,878
Union plan | International equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,365	4,452
Union plan | Fixed income funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Union plan | Fixed income funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,365	4,452
Union plan | Fixed income funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Large U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	29,602	20,378
GK Pension Plan | Large U.S. equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Large U.S. equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	29,602	20,378
GK Pension Plan | Large U.S. equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Small/Mid U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	17,569	12,495
GK Pension Plan | Small/Mid U.S. equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Small/Mid U.S. equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	17,569	12,495
GK Pension Plan | Small/Mid U.S. equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	16,320	25,149
GK Pension Plan | International equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | International equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	16,320	25,149
GK Pension Plan | International equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	38,944	35,627
GK Pension Plan | Fixed income funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Fixed income funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	38,944	35,627
GK Pension Plan | Fixed income funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,677	5,613
GK Pension Plan | Real estate | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
GK Pension Plan | Real estate | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,677	5,613
GK Pension Plan | Real estate | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | Large U.S. equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	39,002	17,756
U.K. Plans | Large U.S. equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | Large U.S. equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	39,002	17,756
U.K. Plans | Large U.S. equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	69,251	102,494
U.K. Plans | International equity funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | International equity funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	69,251	102,494
U.K. Plans | International equity funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | Fixed income funds
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	60,212	53,722
U.K. Plans | Fixed income funds | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | Fixed income funds | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	60,212	53,722
U.K. Plans | Fixed income funds | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | Real estate
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	17,188	0
U.K. Plans | Real estate | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
U.K. Plans | Real estate | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	17,188	0
U.K. Plans | Real estate | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	$ 0	$ 0