DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Jun. 30, 2019 MWh MMBTU	Dec. 31, 2018 MWh MMBTU
Forwards, Futures and Swaps | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	174,575,917	177,750,349
Forwards, Futures and Swaps | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	2,999,616	3,833,490
Options | Natural Gas
Derivative [Line Items]
Contract Volume (mmbtu and mwh) | MMBTU	16,455,000	13,735,405
Options | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	912,033	0
Congestion Revenue Rights | Electricity
Derivative [Line Items]
Contract Volume (mmbtu and mwh)	329,571,344	340,783,089