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GOODWILL AND IDENTIFIABLE INTANGIBLE ASSETS - Narrative (Details) - USD ($)
12 Months Ended
Dec. 31, 2018
Mar. 31, 2019
Jul. 01, 2018
Dec. 31, 2017
Goodwill [Line Items]        
Goodwill $ 734,600,000 $ 734,607,000 $ 731,029,000  
Identifiable intangible assets 26,700,000     $ 28,100,000
Impairment of intangible assets (excluding goodwill) $ 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 $ 730,900,000      
On-Site Energy Production        
Goodwill [Line Items]        
Goodwill 3,600,000      
ELK Utility Operations        
Goodwill [Line Items]        
Goodwill $ 100,000