FAIR VALUE MEASUREMENTS AND DERIVATIVE INSTRUMENTS (Level 3 Fair Value Measurements) (Details) - Level 3 [Member] $ in Millions	Dec. 31, 2016 USD ($) $ / megawatt_hour	Dec. 31, 2015 USD ($) $ / MMBTU $ / megawatt_hour
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Derivative Assets	$ 2	$ 1
Derivative Liabilities	(1)	(3)
Market Approach Valuation Technique [Member] | Forward Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Derivative Assets	2	1
Derivative Liabilities	$ (1)	(2)
Valuation Technique Option Model [Member] | Gas Option Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Derivative Assets		0
Derivative Liabilities		$ (1)
Minimum [Member] | Market Approach Valuation Technique [Member] | Forward Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Market Price | $ / megawatt_hour	20.90	19.20
Minimum [Member] | Valuation Technique Option Model [Member] | Gas Option Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Market Price | $ / MMBTU		2.17
Gas Volatility		31.00%
Maximum [Member] | Market Approach Valuation Technique [Member] | Forward Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Market Price | $ / megawatt_hour	40.00	31.35
Maximum [Member] | Valuation Technique Option Model [Member] | Gas Option Contracts [Member]
Fair Value Inputs Assets Liabilities Quantitative Information [Line Items]
Market Price | $ / MMBTU		2.69
Gas Volatility		58.30%