DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Jun. 30, 2021 MWh MMBTU	Dec. 31, 2020 MMBTU MWh
Forwards, Futures and Swaps | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	252,270,326	146,642,863
Forwards, Futures and Swaps | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	10,375,299	9,435,830
Options | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	35,515,000	14,140,000
Options | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	411,200	0
Congestion Revenue Rights | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	259,232,639	266,091,470