GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - NARRATIVE (Details) - USD ($)	3 Months Ended		12 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill impairment			$ 0	$ 3,637,000
Goodwill			706,960,000	702,070,000	$ 734,607,000
Impairment of intangible assets			0	4,800,000	0
Amortization expense			5,400,000	6,100,000	$ 3,500,000
Disposal Group, Disposed of by Sale, Not Discontinued Operations | MTF and ACB
Goodwill [Line Items]
Goodwill impairment				3,600,000
ETG Utility Operations
Goodwill [Line Items]
Goodwill impairment	$ 0	$ 0
Goodwill			700,200,000	700,200,000
Corporate Services and Other Operations
Goodwill [Line Items]
Goodwill			$ 6,800,000	1,900,000
ELK Utility Operations
Goodwill [Line Items]
Goodwill				$ 100,000
Minimum
Goodwill [Line Items]
Useful life of finite-lived intangible assets (in years)			2 years
Maximum
Goodwill [Line Items]
Useful life of finite-lived intangible assets (in years)			20 years