DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Sep. 30, 2020 MMBTU MWh	Dec. 31, 2019 MMBTU MWh
Forwards, Futures and Swaps | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	159,168,216	131,896,159
Forwards, Futures and Swaps | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	9,925,993	18,675,852
Options | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	43,210,000	14,720,000
Options | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	584,800	0
Congestion Revenue Rights | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	275,838,390	308,467,999