DERIVATIVES (Volumes of Outstanding Derivative Contracts) (Details)	Dec. 31, 2020 MWh MMBTU	Dec. 31, 2019 MMBTU MWh
Forwards, Futures and Swaps | Natural Gas (MMBtus)
Derivative [Line Items]
Contract Volume	146,642,863	131,896,159
Forwards, Futures and Swaps | Electricity (Megawatt-hours)
Derivative [Line Items]
Contract Volume | MWh	9,435,830	18,675,852
Options | Natural Gas (MMBtus)
Derivative [Line Items]
Contract Volume	14,140,000	14,720,000
Options | Electricity (Megawatt-hours)
Derivative [Line Items]
Contract Volume	0	0
Congestion revenue rights | Electricity (Megawatt-hours)
Derivative [Line Items]
Contract Volume | MWh	266,091,470	308,467,999