RESTRUCTURING - Summary of Restructuring Charges (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 1,131	$ 469	$ 631	$ 368	$ 1,541	$ 2,588	$ 0	$ 0	$ 2,599	$ 4,129	$ 16,917
Asset impairment charges									1,100	1,600	800
Redundancy and employee severance and benefit arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring charges									1,575	2,228	2,827
Asset impairments
Restructuring Cost and Reserve [Line Items]
Asset impairment charges									1,124	1,601	522
Consultants engaged in restructuring activities
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0	0	12,100
Lease loss reserves
Restructuring Cost and Reserve [Line Items]
Restructuring reserve, accrual adjustment									(100)	300	$ 1,468
Restructuring Plan 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges									2,599	4,204
Restructuring Plan 2018 | Redundancy and employee severance and benefit arrangements
Restructuring Cost and Reserve [Line Items]
Restructuring charges									1,575	2,228
Restructuring Plan 2018 | Asset impairments
Restructuring Cost and Reserve [Line Items]
Restructuring charges									1,124	1,636
Restructuring Plan 2018 | Lease loss reserves
Restructuring Cost and Reserve [Line Items]
Restructuring charges									$ (100)	$ 340