REVENUE - Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Contract asset (current)	$ 20,864	$ 16,109
Increase / (decrease) in contract asset (current)	4,755
Contract liability (current)	101,299	87,188
Increase / (decrease) in contract liability (current)	14,111
Contract liability (long-term)	23,129	$ 21,584
Increase / (decrease) in contract liability (long-term)	1,545
Revenue recognized previously included in current contract liabilities	49,900
Revenue recognized previously included in current contract liabilities at the beginning of the year	$ 29,800