DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Mar. 31, 2021 MWh MMBTU	Dec. 31, 2020 MMBTU MWh
Forwards, Futures and Swaps | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	219,348,075	146,642,863
Forwards, Futures and Swaps | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	11,127,600	9,435,830
Options | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	17,080,000	14,140,000
Options | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	588,800	0
Congestion Revenue Rights | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	264,206,953	266,091,470