SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Revenue Recognition (Details)	12 Months Ended
	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Retail revenue period for billing postpaid customers in advance	1 month
Expected amortization period, when the Company utilizes the practical expedient and expenses the costs as incurred	true
Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Term of power purchase agreements with various customers (in years)	10 years
Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Term of power purchase agreements with various customers (in years)	25 years
Renewable Energy | Minimum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization period	2 years
Renewable Energy | Maximum
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Amortization period	6 years