FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Derivative Volumes) (Details) $ in Millions, BBtu in Billions	Dec. 31, 2019 USD ($) BBtu GWh	Dec. 31, 2018 BBtu GWh
Interest Rate Swap
Derivative Instruments, Gain (Loss) [Line Items]
Derivative liability, notional amount | $	$ 6
Power Contracts GWh
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, nonmonetary notional amount | GWh	4,740	1,743
Gas Contracts BBtu
Derivative Instruments, Gain (Loss) [Line Items]
Derivative, nonmonetary notional amount | BBtu	122,779	146,933