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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)
 12 Months Ended
Dec. 31, 2018
Minimum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Amortization period 2 years
Maximum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Amortization period 6 years
Renewable Energy | Minimum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Term of power purchase agreements with various customers (in years) 10 years
Renewable Energy | Maximum  
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]  
Term of power purchase agreements with various customers (in years) 25 years