GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Jul. 01, 2018
Goodwill [Line Items]
Goodwill	$ 705,707,000		$ 705,707,000		$ 734,607,000	$ 700,286,000
Identifiable intangible assets	26,000,000.0		26,000,000.0		28,100,000
Impairment of intangible assets (excluding goodwill)	0	$ 0	$ 0	$ 0
Minimum
Goodwill [Line Items]
Useful life of finite-lived intangible assets			2 years
Maximum
Goodwill [Line Items]
Useful life of finite-lived intangible assets			20 years
ETG Utility Operations
Goodwill [Line Items]
Goodwill	700,200,000		$ 700,200,000		730,900,000
On-Site Energy Production
Goodwill [Line Items]
Goodwill	3,600,000		3,600,000		3,600,000
ELK Utility Operations
Goodwill [Line Items]
Goodwill	100,000		100,000		$ 100,000
Corporate and Other
Goodwill [Line Items]
Goodwill	$ 1,800,000		$ 1,800,000