DERIVATIVES (Volumes of Outstanding Derivative Contracts, in Megawatt Hours Unless Otherwise Specified) (Details)	Mar. 31, 2018 MWh MMBTU	Dec. 31, 2017 MWh MMBTU
Forwards, Futures and Swaps | Natural gas
Derivative [Line Items]
Contract Volume | MMBTU	184,948,051	228,768,745
Forwards, Futures and Swaps | Electricity
Derivative [Line Items]
Contract Volume | MWh	2,602,376	2,872,013
Options | Natural gas
Derivative [Line Items]
Contract Volume | MMBTU	31,481,247	60,736,806
Congestion Revenue Rights | Electricity
Derivative [Line Items]
Contract Volume | MWh	304,484,831	312,272,177