GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - SUMMARY OF CHANGES IN GOODWILL (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Goodwill [Roll Forward]
Goodwill, Beginning Balance	$ 734,607	$ 3,578
ETG and ELK Acquisition-related Working Capital Settlement	(15,600)	0
ETG and ELK Fair Value Adjustments During Measurement Period	(15,143)	(25,218)
Reclassified to Assets Held for Sale	(3,637)	0
Goodwill, Ending Balance	702,070	734,607
AEP Acquisition
Goodwill [Roll Forward]
Goodwill from Acquisition	1,843	0
ETG and ELK Acquisition
Goodwill [Roll Forward]
Goodwill from Acquisition	$ 0	$ 756,247

Business Acquisition, Provisional Accounting Incomplete, Adjustment, Working Capital Settlement

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Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of transfers into (out of) an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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