FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Derivative Volumes) (Details) $ in Millions	Dec. 31, 2016 USD ($) GWh BTU	Dec. 31, 2015 GWh BTU
Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Liability, Notional Amount | $	$ 23
Power Contracts (in GWh) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives Volumes | GWh	2,610	1,752
Gas Contracts (in BBtu) [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivatives Volumes | BTU	12,355,000,000,000	17,214,000,000,000