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Derivative Instruments and Hedging Activities (Balance Sheet Disclosures) (Details) (USD $)
In Millions, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term $ 41 $ 53
Derivative Assets, Long Term 22 37
Derivative Assets 63 90
Derivative Liabilities, Long Term 68 118
Derivative Liabilities 68 118
Margin and collateral deposits 89 61
Midwest Generation LLC
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 1 2
Derivative Liabilities, Short-term 5 3
Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets 63 90
Electricity contracts | Midwest Generation LLC
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets 1 2
Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets 0 0
Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets 0 0
Gross Derivative Contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 163 155
Derivative Assets, Long Term 46 53
Derivative Liabilities, Short-term 122 109
Derivative Liabilities, Long Term 91 134
Gross Derivative Contract | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 135 120
Derivative Assets, Long Term 43 52
Gross Derivative Contract | Electricity contracts | Midwest Generation LLC
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 10 12
Derivative Liabilities, Short-term 14 13
Gross Derivative Contract | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 27 33
Derivative Assets, Long Term 3 1
Gross Derivative Contract | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 1 2
Derivative Assets, Long Term 0 0
Netting and Collateral Received
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term (122) (102)
Derivative Assets, Long Term (24) (16)
Derivative Liabilities, Short-term (122) (109)
Derivative Liabilities, Long Term (23) (16)
Netting and Collateral Received | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term (94) (67)
Derivative Assets, Long Term (21) (15)
Netting and Collateral Received | Electricity contracts | Midwest Generation LLC
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term (9) (10)
Derivative Liabilities, Short-term (9) (10)
Netting and Collateral Received | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term (27) (33)
Derivative Assets, Long Term (3) (1)
Netting and Collateral Received | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term (1) (2)
Derivative Assets, Long Term 0 0
Net Derivative Contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 41 53
Derivative Assets, Long Term 22 37
Derivative Liabilities, Short-term 0 0
Derivative Liabilities, Long Term 68 118
Net Derivative Contract | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 41 53
Derivative Assets, Long Term 22 37
Net Derivative Contract | Electricity contracts | Midwest Generation LLC
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 1 2
Derivative Liabilities, Short-term 5 3
Net Derivative Contract | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 0 0
Derivative Assets, Long Term 0 0
Net Derivative Contract | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Assets, Short-term 0 0
Derivative Assets, Long Term 0 0
Subject to Master Netting Agreement
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities 0 0
Subject to Master Netting Agreement | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities 0 0
Subject to Master Netting Agreement | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities 0 0
Subject to Master Netting Agreement | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities 0 0
Subject to Master Netting Agreement | Gross Derivative Contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 122 109
Derivative Liabilities, Long Term 23 16
Subject to Master Netting Agreement | Gross Derivative Contract | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 96 71
Derivative Liabilities, Long Term 20 15
Subject to Master Netting Agreement | Gross Derivative Contract | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 25 36
Derivative Liabilities, Long Term 3 1
Subject to Master Netting Agreement | Gross Derivative Contract | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 1 2
Derivative Liabilities, Long Term 0 0
Subject to Master Netting Agreement | Netting and Collateral Received
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term (122) (109)
Derivative Liabilities, Long Term (23) (16)
Subject to Master Netting Agreement | Netting and Collateral Received | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term (96) (71)
Derivative Liabilities, Long Term (20) (15)
Subject to Master Netting Agreement | Netting and Collateral Received | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term (25) (36)
Derivative Liabilities, Long Term (3) (1)
Subject to Master Netting Agreement | Netting and Collateral Received | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term (1) (2)
Derivative Liabilities, Long Term 0 0
Subject to Master Netting Agreement | Net Derivative Contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 0
Derivative Liabilities, Long Term 0 0
Subject to Master Netting Agreement | Net Derivative Contract | Electricity contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 0
Derivative Liabilities, Long Term 0 0
Subject to Master Netting Agreement | Net Derivative Contract | Natural gas contracts
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 0
Derivative Liabilities, Long Term 0 0
Subject to Master Netting Agreement | Net Derivative Contract | Coal contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 0
Derivative Liabilities, Long Term 0 0
Not Subject to Master Netting Agreement
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities 68 [1] 118 [1]
Not Subject to Master Netting Agreement | Gross Derivative Contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 [1] 0 [1]
Derivative Liabilities, Long Term 68 [1] 118 [1]
Not Subject to Master Netting Agreement | Netting and Collateral Received
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 [1] 0 [1]
Derivative Liabilities, Long Term 0 [1] 0 [1]
Not Subject to Master Netting Agreement | Net Derivative Contract
    
Derivative, Fair Value, Net [Abstract]    
Derivative Liabilities, Short-term 0 [1] 0 [1]
Derivative Liabilities, Long Term 68 [1] 118 [1]
Net accounts receivable
    
Derivative, Fair Value, Net [Abstract]    
Derivative collateral, net accounts receivable $ 65 $ 44
[1] EME's interest rate swaps are not subject to master netting agreements and do not require EME to post collateral.