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FAIR VALUE MEASUREMENTS 4 (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended
Jun. 30, 2013
Jun. 30, 2012
Jun. 30, 2013
Jun. 30, 2012
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance at beginning of period $ 58 $ 21 $ 61 $ 23
Realized and unrealized gains (losses) (2) 5 (3) 7
Allocated transmission instruments       1
Settlements (9) (13) (11) (18)
Balance at end of period 47 13 47 13
Change in unrealized gains (losses) relating to instruments still held at period end (1)   (1)  
CRR Auction Rate Per MWh, Minimum (8)   (8)  
CRR Auction Rate Per MWh, Maximum $ 8   $ 8