FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Potential Offset of Counterparty Netting and Cash Collateral) (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Energy Derivative Contracts
Derivative Assets
Gross Amount Recognized in the Balance Sheets	$ 9	$ 12
Counterparty Netting of Energy Contracts	6	10
Cash Collateral Received	0	0
Net Amount	3	2
Derivative Liabilities
Gross Amount Recognized in the Balance Sheets	(48)	(27)
Counterparty Netting of Energy Contracts	(6)	(10)
Cash Collateral Posted	0	0
Net Amount	$ (42)	(17)
Interest Rate Swap
Derivative Liabilities
Gross Amount Recognized in the Balance Sheets		(1)
Counterparty Netting of Energy Contracts		0
Cash Collateral Posted		0
Net Amount		$ (1)

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