REVENUE - Contract Assets and Contract Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Revenue from Contract with Customer [Abstract]
Contract asset (current)	$ 18,656	$ 16,109
Increase / (decrease) in contract asset (current)	2,547
Contract liability (current)	84,199	87,188
Increase / (decrease) in contract liability (current)	(2,989)
Contract liability (long-term)	21,725	$ 21,584
Increase / (decrease) in contract liability (long-term)	141
Revenue recognized previously included in current contract liabilities	80,400
Revenue recognized previously included in current contract liabilities at the beginning of the year	$ 44,900