REVENUE - Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Contract asset (current)	$ 15,490	$ 16,109
Increase / (decrease) in contract asset (current)	(619)
Contract asset (long-term)	0	0
Increase / (decrease) in contract assets (long-term)	0
Contract liability (current)	102,721	87,188
Increase / (decrease) in contract liability (current)	15,533
Contract liability (long-term)	25,747	$ 21,584
Increase / (decrease) in contract liability (long-term)	4,163
Revenue recognized previously included in current contract liabilities	18,700
Revenue recognized previously included in current contract liabilities at beginning of the year	$ 13,000