COMPREHENSIVE INCOME - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	$ (294,939)	$ (277,297)
Other comprehensive income (loss) before reclassifications	(53,318)	12,034
Reclassified from AOCI to net income (loss)	(998)	1,521
Amounts reclassified from AOCI to advanced billings on contracts		(197)
Other comprehensive (loss) income	(54,316)	13,358
Ending balance	(338,262)	(294,939)
Accumulated Other Comprehensive (Loss) Income
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	1,926	(11,432)
Ending balance	(52,390)	1,926
Currency translation (loss) gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	5,743	(10,834)
Other comprehensive income (loss) before reclassifications	(53,318)	13,401
Reclassified from AOCI to net income (loss)	0	3,176
Amounts reclassified from AOCI to advanced billings on contracts		0
Other comprehensive (loss) income	(53,318)	16,577
Ending balance	(47,575)	5,743
Net unrealized gain (loss) on derivative instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	0	1,362
Other comprehensive income (loss) before reclassifications	0	(1,367)
Reclassified from AOCI to net income (loss)	0	202
Amounts reclassified from AOCI to advanced billings on contracts		(197)
Other comprehensive (loss) income	0	(1,362)
Ending balance	0	0
Net unrecognized loss related to benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Beginning balance	(3,817)	(1,960)
Other comprehensive income (loss) before reclassifications	0	0
Reclassified from AOCI to net income (loss)	(998)	(1,857)
Amounts reclassified from AOCI to advanced billings on contracts		0
Other comprehensive (loss) income	(998)	(1,857)
Ending balance	$ (4,815)	$ (3,817)