DERIVATIVES (Volumes of Outstanding Derivative Contracts) (Details)	Dec. 31, 2018 MWh MMBTU	Dec. 31, 2017 MWh MMBTU
Forwards and Swaps | Natural Gas (MMBtus)
Derivative [Line Items]
Contract Volume | MMBTU	177,750,349	228,768,745
Forwards and Swaps | Electricity (Megawatt-hours)
Derivative [Line Items]
Contract Volume | MWh	3,833,490	2,872,013
Options | Natural Gas (MMBtus)
Derivative [Line Items]
Contract Volume | MMBTU	13,735,405	60,736,806
Congestion revenue rights | Electricity (Megawatt-hours)
Derivative [Line Items]
Contract Volume | MWh	340,783,089	312,272,177