PENSION AND OTHER POSTRETIREMENT BENEFITS (Schedule of Fair Value Assumptions of Plan Assets) (Details) - USD ($) $ in Thousands	Mar. 28, 2021	Dec. 27, 2020
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	$ 312,429	$ 305,983
Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,975	1,487
Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	306,454	304,496
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	5,975	1,487
Cash and cash equivalents | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,975	1,487
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
Large U.S. equity funds | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	3,339	3,100
Large U.S. equity funds | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	31,190	29,602
Large U.S. equity funds | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	43,792	39,002
Large U.S. equity funds | Level 1 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Large U.S. equity funds | Level 1 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Large U.S. equity funds | Level 1 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Large U.S. equity funds | Level 2 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	3,339	3,100
Large U.S. equity funds | Level 2 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	31,190	29,602
Large U.S. equity funds | Level 2 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	43,792	39,002
Large U.S. equity funds | Level 3 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Large U.S. equity funds | Level 3 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Large U.S. equity funds | Level 3 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Small/Mid U.S. equity funds | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	451	392
Small/Mid U.S. equity funds | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	16,602	17,569
Small/Mid U.S. equity funds | Level 1 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Small/Mid U.S. equity funds | Level 1 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Small/Mid U.S. equity funds | Level 2 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	451	392
Small/Mid U.S. equity funds | Level 2 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	16,602	17,569
Small/Mid U.S. equity funds | Level 3 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Small/Mid U.S. equity funds | Level 3 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
International equity funds | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	2,013	1,874
International equity funds | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	15,682	16,320
International equity funds | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	68,524	69,251
International equity funds | Level 1 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
International equity funds | Level 1 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
International equity funds | Level 1 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
International equity funds | Level 2 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	2,013	1,874
International equity funds | Level 2 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	15,682	16,320
International equity funds | Level 2 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	68,524	69,251
International equity funds | Level 3 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
International equity funds | Level 3 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
International equity funds | Level 3 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Fixed income securities | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,055	5,365
Fixed income securities | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	37,860	38,944
Fixed income securities | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	59,076	60,212
Fixed income securities | Level 1 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Fixed income securities | Level 1 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Fixed income securities | Level 1 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Fixed income securities | Level 2 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,055	5,365
Fixed income securities | Level 2 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	37,860	38,944
Fixed income securities | Level 2 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	59,076	60,212
Fixed income securities | Level 3 | Union Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Fixed income securities | Level 3 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Fixed income securities | Level 3 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Real estate | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,163	5,677
Real estate | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	17,707	17,188
Real estate | Level 1 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Real estate | Level 1 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Real estate | Level 2 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	5,163	5,677
Real estate | Level 2 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	17,707	17,188
Real estate | Level 3 | GK Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	0	0
Real estate | Level 3 | U.K. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Fair value of plan assets	$ 0	$ 0