GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - SUMMARY OF CHANGES IN GOODWILL (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Goodwill [Roll Forward]
Beginning Balance	$ 702,070,000	$ 734,607,000
ETG and ELK Acquisition-related Working Capital Settlement at the ETG and ELK Utility Operations segments	0	(15,600,000)
ETG and ELK Fair Value Adjustments During Measurement Period at the ETG and ELK Utility Operations segments	0	(15,143,000)
Impairment Charge at the On-Site Energy Production segment	0	(3,637,000)
Ending Balance	706,960,000	702,070,000
AEP Acquisition
Goodwill [Roll Forward]
Goodwill from Acquisition at Corporate & Services segment	0	1,843,000
EnerConnex, LLC
Goodwill [Roll Forward]
Goodwill from Acquisition at Corporate & Services segment	$ 4,890,000	$ 0