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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - PROPERTY PLANT AND EQUIPMENT (Details) - USD ($)
$ in Thousands
1 Months Ended 12 Months Ended
Sep. 30, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Public Utility, Property, Plant and Equipment [Line Items]        
Composite rate of depreciation   2.29% 2.25% 2.32%
Depreciation and amortization   $ 85,701 $ 81,841 $ 72,748
Property Classifications        
Property, plant and equipment - wind equipment, at cost   3,304,491 3,211,070  
Accumulated depreciation and amortization   (653,442) (601,329)  
Property, plant and equipment, net   2,651,049 2,609,741  
Regulated Operation | Distribution facilities        
Property Classifications        
Property, plant and equipment - wind equipment, at cost   $ 2,151,249 1,952,697  
Regulated Operation | Distribution facilities | Minimum        
Property Classifications        
Estimated Useful Lives   38 years    
Regulated Operation | Distribution facilities | Maximum        
Property Classifications        
Estimated Useful Lives   74 years    
Regulated Operation | Transmission facilities        
Property Classifications        
Property, plant and equipment - wind equipment, at cost   $ 295,692 294,586  
Regulated Operation | Transmission facilities | Minimum        
Property Classifications        
Estimated Useful Lives   35 years    
Regulated Operation | Transmission facilities | Maximum        
Property Classifications        
Estimated Useful Lives   56 years    
Regulated Operation | Storage facilities        
Property Classifications        
Property, plant and equipment - wind equipment, at cost   $ 79,470 78,245  
Regulated Operation | Storage facilities | Minimum        
Property Classifications        
Estimated Useful Lives   34 years    
Regulated Operation | Storage facilities | Maximum        
Property Classifications        
Estimated Useful Lives   47 years    
Unregulated Operation | Solar property        
Property Classifications        
Property, plant and equipment - wind equipment, at cost   $ 720,562 587,345  
Unregulated Operation | Solar property | Minimum        
Property Classifications        
Estimated Useful Lives   20 years    
Unregulated Operation | Solar property | Maximum        
Property Classifications        
Estimated Useful Lives   25 years    
Unregulated Operation | Wind property        
Property Classifications        
Property, plant and equipment - wind equipment, at cost [1]   $ 0 244,764  
Unregulated Operation | Wind property | Minimum        
Property Classifications        
Estimated Useful Lives [1]   17 years    
Unregulated Operation | Wind property | Maximum        
Property Classifications        
Estimated Useful Lives [1]   25 years    
Unregulated Operation | Midstream property        
Property Classifications        
Estimated Useful Lives [2]   30 years    
Property, plant and equipment - wind equipment, at cost [2]   $ 6,747 0  
Unregulated Operation | All other property        
Property Classifications        
Property, plant and equipment - wind equipment, at cost   $ 50,771 $ 53,433  
Unregulated Operation | All other property | Minimum        
Property Classifications        
Estimated Useful Lives   5 years    
Unregulated Operation | All other property | Maximum        
Property Classifications        
Estimated Useful Lives   35 years    
September 2016 Base Rate Case        
Public Utility, Property, Plant and Equipment [Line Items]        
Composite rate of depreciation 2.40%      
[1] Reclassified to assets held for sale. See Note 2. Summary of Significant Accounting Policies, for more information.
[2] Includes activity related to the acquisition of IEC by Adelphia. For fiscal 2017, Midstream property was originally included in All other property.