DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Mar. 31, 2020 MWh MMBTU	Dec. 31, 2019 MMBTU MWh
Forwards, Futures and Swaps | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	138,102,835	131,896,159
Forwards, Futures and Swaps | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MWh	49,291,087	18,675,852
Options | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	7,760,000	14,720,000
Options | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	4,414,400	0
Congestion Revenue Rights | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MWh	298,648,904	308,467,999