FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Derivative Volumes) (Details) $ in Millions, BTU in Billions	12 Months Ended
	Dec. 31, 2017 USD ($) GWh BTU	Dec. 31, 2016 GWh BTU
Power Contracts GWh
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | GWh	2,589	2,610
Gas Contracts BBtu (1)
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure | BTU	137,952	12,355
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount | $	$ 18