FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Derivative Volumes) (Details) $ in Millions	Dec. 31, 2018 USD ($) GWh BTU	Dec. 31, 2017 GWh BTU
Power Contracts GWh
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount | GWh	1,743	2,589
Gas Contracts BBtu
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, Nonmonetary Notional Amount | BTU	146,933,000,000,000	137,952,000,000,000
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount | $	$ 12