REVENUE - Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Revenue from Contract with Customer [Abstract]
Contract asset (current)	$ 16,109	$ 16,245
Increase / (decrease) in contract asset (current)	(136)
Contract asset (long-term)	0	0
Increase / (decrease) in contract asset (long-term)	0
Contract liability (current)	87,188	79,305
Increase / (decrease) in contract liability (current)	7,883
Contract liability (long-term)	21,584	$ 9,779
Increase / (decrease) in contract liability (long-term)	11,805
Revenue recognized previously included in current contract liabilities	107,700
Revenue recognized previously included in current contract liabilities at beginning of the year	$ 57,800