RESTRUCTURING CHARGES - Restructuring charges (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 11,981	$ 6,976	$ 5,830
Costs to exit duplicative contracts
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	0	0	229
Workforce reduction
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	11,885	6,171	3,599
Other restructuring costs
Restructuring Cost and Reserve [Line Items]
Total restructuring charges	$ 96	$ 805	$ 2,002