COMPREHENSIVE INCOME - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	$ (338,700)	$ (323,650)	$ (294,939)	$ (343,035)	$ (317,852)	$ (277,297)	$ (294,939)	$ (277,297)
Other comprehensive income (loss) before reclassifications	(22,916)	(4,095)	2,380	21,433	(8,168)	9,082
Reclassified from AOCI to net income (loss)	(246)	(246)	(246)	(515)	2,640	(132)
Amounts reclassified from AOCI to advanced billings on contracts				(197)
Other comprehensive (loss) income	(23,162)	(4,341)	2,134	20,721	(5,528)	8,950	(25,369)	24,143
Stockholders' equity, ending balance	(320,778)	(338,700)	(323,650)	(290,077)	(343,035)	(317,852)	(320,778)	(290,077)
Accumulated Other Comprehensive Income (Loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(281)	4,060	1,926	(8,010)	(2,482)	(11,432)	1,926	(11,432)
Stockholders' equity, ending balance	(23,443)	(281)	4,060	12,711	(8,010)	(2,482)	(23,443)	12,711
Currency translation (loss) gain
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	4,028	8,123	5,743	(5,377)	(574)	(10,834)	5,743	(10,834)
Other comprehensive income (loss) before reclassifications	(22,916)	(4,095)	2,380	21,433	(7,979)	10,260
Reclassified from AOCI to net income (loss)	0	0	0	0	3,176	0
Amounts reclassified from AOCI to advanced billings on contracts				0
Other comprehensive (loss) income	(22,916)	(4,095)	2,380	21,433	(4,803)	10,260
Stockholders' equity, ending balance	(18,888)	4,028	8,123	16,056	(5,377)	(574)	(18,888)	16,056
Net unrealized gain (loss) on derivative instruments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance				197	408	1,362		1,362
Other comprehensive income (loss) before reclassifications				0	(189)	(1,178)
Reclassified from AOCI to net income (loss)				0	(22)	224
Amounts reclassified from AOCI to advanced billings on contracts				(197)
Other comprehensive (loss) income				(197)	(211)	(954)
Stockholders' equity, ending balance				0	197	408		0
Net unrecognized loss related to benefit plans (net of tax)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Stockholders' equity, beginning balance	(4,309)	(4,063)	(3,817)	(2,830)	(2,316)	(1,960)	(3,817)	(1,960)
Other comprehensive income (loss) before reclassifications	0	0	0	0	0	0
Reclassified from AOCI to net income (loss)	(246)	(246)	(246)	(515)	(514)	(356)
Amounts reclassified from AOCI to advanced billings on contracts				0
Other comprehensive (loss) income	(246)	(246)	(246)	(515)	(514)	(356)
Stockholders' equity, ending balance	$ (4,555)	$ (4,309)	$ (4,063)	$ (3,345)	$ (2,830)	$ (2,316)	$ (4,555)	$ (3,345)