ACCUMULATED OTHER COMPREHENSIVE INCOME - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	$ 2,418,735	$ 2,346,545
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	284	0
Other comprehensive income	284	0
Gross amounts reclassified within AOCI	0	0
Balance, end of period	2,403,251	2,258,382
Total Post- Retirement Benefits Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	2,001	2,082
Gross amounts reclassified within AOCI	0	0
Balance, end of period	2,285	2,082
Foreign Currency Translation Adjustments
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	0	(45,205)
Gross amounts reclassified within AOCI	0	25,155
Balance, end of period	0	(20,050)
Non-controlling Interest - Former Principal Equityholders Reclassification
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	0	25,155
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Other comprehensive income	0	0
Gross amounts reclassified within AOCI	0	(25,155)
Balance, end of period	0	0
Total
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of period	2,001	(17,968)
Other comprehensive income	284
Balance, end of period	2,285	(17,968)
Accumulated Defined Benefit Plans Adjustment Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	284
Other comprehensive income	284	0
Accumulated Foreign Currency Adjustment Including Portion Attributable to Noncontrolling Interest [Member]
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0
Other comprehensive income	$ 0	$ 0